Army creation guide

Leadership

The first and perhaps most important part of your army is its leadership. While it's possible to have an army without any leadership, an effective army generally has at least a decent structure of ranks, leaders, and thus organization.
Most armies have 1 absolute leader who's in control of everything, whether that's a king, a president, or a general doesn't matter, and that's something which depends on your story anyway, but the personality of this 1 person will affect much of the army and its structure.

A rash and violent leader might have a huge army full of lesser trained soldiers, and thinks numbers alone will win the fight. A cruel leader might have divisions of cannon fodder, divisions purely meant to die and weaken the enemies despite the cost of their lives. A thoughtful leader might carefully consider tactics, various divisions, how to use strengths of one division against the weaknesses of the enemy, and so on.

Of course in real life that 1 absolute leader is usually accompanied by various advisers for various aspects of war, ranging from tactics to intelligence, and from espionage to treating the wounded. But how many advisers your leader has, and whether he or she listens to them all, is again something which depends on what kind of leader they are.

Besides the type of advisers you have for your leader, the type of leader you have will mostly affect tactics and use of troops, but it could also affect what type of weaponry is used. Is your leader okay with chemical warfare? What about nuclear? Or does your leader stick to certain rules? Perhaps your story is set in a time where those technologies were not even possible to be considered, but does your leader actively seek out new means of destroying the enemy, or does your leader stick to perfecting the techniques they know?

Ranks

Besides picking advisers you'll also have to pick ranks, ranging from various types of lower leadership to ranks for various types of soldiers. It is not a good idea to simply copy the ranks we use in armies around the world today or those we used in history. While those will form a great basis, you need to think about how your army would be best organized, and this will largely depend on the culture of your soldiers.

Brutish orcs might not work well together, and have a need for hundreds of smaller divisions with a strong leader for each to keep all orcs in control. Calm elves might not need many leaders at all, they might be capable of following orders even if they're just one elf within a division of 5000.
The types of ranks, and how many of them you have is thus also dependent on the culture behind your army. Some might need a lot of ranks in order to be able to please fighters by promoting them, others might need very few ranks, perhaps only 1 general, 1 rank for the lower leaders who pass on the commands of the general, and 1 final rank for all normal soldiers.

Ranks will also mean you have to think about how much control each rank has. Who and what divisions does a captain control? Is a medic the same rank as a soldier, or does one have the power to command the other?

Lastly, you'll have to think about names for your ranks. Your fictional universe might be closely based on our own, and work perfectly fine with the terms we use today, but if it's different you're better of picking different names. Some are easier to change, like changing a medic to a doctor, healer, mender, physician, or even shaman. Others are more difficult to change, but if you're stuck the military rank name generator on my other site might be of help.

Divisions

So you've picked your leadership, and you have a rough idea of what your rank system will look like, now it's time to think about the divisions within your army. Some are quite obvious, like a naval division, a medical division, and a cavalry division. These again depend on what you need in your story, and what fits in your universe. A world without medicine, or a culture which sees shame in being saved from death in the battlefield likely won't have a medical division, you can't have a naval division without ships, and so on.

But some divisions might be less obvious, like a veterinary divisions to take care of any wounded animals you may use, ranging from horses to attack dogs, and birds used to deliver messages. You may also wish to have an administrative division, tunneling divisions, mountain divisions, training divisions, public affairs divisions, local relationship divisions, and so on.
There's an incredible range of divisions you could use in your army. In order to decide which you need or want it's best to both do a little research to see what we've used throughout history, and to know the types of units you have in your army.

Also keep in mind what enemy your army is supposed to fight in your story. If your enemy army has users of magic or airplanes, your army may be forced to create a division dedicated to taking those out, even if they don't have the proper means to do so.

If you need help coming up with divisions, try out the military division name generator on my other site.

Tactics

A big and complicated part of an army is the way it fights. Obviously there's an enormous amount of factors which have to be taken into account when you consider tactics, but the point in this guide is focused on considering the types of units and divisions you pick with battles in mind, which brings me to my main point: experience.

An army which has fought many battles will likely know far more about how to engage in battle, and thus about tactics, than an army which has never fought at all. Of course this doesn't mean a fresh army won't be able to use advanced tactics, a leader with a keen eye for tactics might still be able to come up with clever ways to defeat the enemy. However, an experienced army will not only be better at executing certain tactics, especially more complicated ones, but they will likely also have more faith in their leaders and their decisions.

Other than that it's really difficult to create original tactics, not only does it depend on the units your army has, the enemy has, the type of terrain, weather, weapon types, available siege engines, available troops, troop experience, leader experience, troop confidence, morale, and far more, it also depends on how good of a tactician you yourself are. You may have the perfect army, but if you're not a great tactician yourself, it'll be very difficult to write them for your story. The best thing you can do is do research, and not just into armies which are similar to you. There's a wealth of knowledge on the internet about tactics, both tactics we use today and those used by leaders throughout history. If nothing else, you can replicate and slightly alter battles from history to your own universe, but I'm sure doing the research will give you plenty of inspiration to at least take bits and pieces from various battles, and applying them to your own as well.

Special forces

I'm sure some of you are tempted to add various special forces to your army, which is perfectly fine, and a realistic thing to do, but it's important to consider what special forces require, which is training, or in other words experience.
In order to train ordinary forces to become special forces you need means to do so. For some forces this means having teachers with battle experience to pass down onto these new soldiers, for other forces it may be something as simple as hunting experience, which means being able to stealthily track an animal, and take it down from a distance.

By making sure your special forces have the means to become who they are, you not only make sure your army is more realistic, you also enrich your world and the culture behind your army. For example, consider the cultural differences between the ancient Spartans and the Shaolin monks, or between the samurai and ninjas.

Armor & Weapons

In terms of keeping an army realistic you won't have to worry all that much about armor and weaponry, but there are some things you may wish to consider. One is the cost of manufacturing the armor. In a fantasy setting you could easily explain those through various means, but in a more realistic story this could play an important factor.

The simpler the armor the less it'll cost to create, so while you may have an intimidating armor design in mind with metallic masks, decorated breastplates, or gilded decorations, these designs aren't very realistic for an army supported by very little. When armor needs to be created at a fast rate, and in large numbers to support an increase in army size, you'll also want to stick to basic, easy to craft designs.

On the other hand this doesn't mean you can't have awesome designs at all. Higher ranked soldiers may be given more decorative armor pieces as a reward and sign of status, and some individual soldiers may have saved up enough money to buy a more special set of armor.
In fantasy settings it's easier to explain why all soldiers have more decorative armor pieces, perhaps you have the most skilled metalworkers in the world, maybe you're able to use magic, or maybe you have special forges and metalworking techniques which allow for mass production of armor.

Weaponry gets a little more complicated than armor. While weaponry also relies on the cost of manufacturing, it also relies on training, reliability, effectiveness against the enemy, and more. For example, a sword is generally a great choice in medieval battle, but it's expensive to make, and it requires training to be used effectively. An axe or spear on the other hand is much cheaper to create, and requires less training and experience to use, but if you're up against a swordsman you're likely in a disadvantageous position.

The same applies to guns and pretty much any type of weaponry. One type of rifle may be the perfect choice in a rural area, at the same time it may be a terrible choice in jungles or other natural areas due to water, sand, and other junk getting stuck within the rifle.
Of course this is less of an issue in fantasy stories where you can make up your own weapons, but having a weapon which is the best in every situation can come across as unrealistic.

If you do wish to use more decorative armor sets, the armor creator might be of help when you need a little inspiration.

Population & Economy

If you want to really keep things realistic, you'll have to take into account the population and the economy supporting your army. Armies cost an incredible amount of resources, ranging from payment and food for the soldiers to resources for weaponry and transport, and so much more. This has to come from somewhere, which means your country either has to have enormous stockpiles or a big enough population to build, grow, and otherwise provide those resources.

Whether you have to gather the resources or if you have them stored will affect how many soldiers you can recruit into your army. If you don't have large food stocks you will need plenty of farmers to grow food, and/or hunters to hunt food for not just the soldiers, but for the entire population. If you don't have many ores in stock, you will need miners and metalworkers to create your weaponry.

The longer a war lasts the lower any stocks you had will get, and you might risk running out of both stocks and population due to the war itself. While this all isn't something that is laid out in great detail in most stories, it is something to keep in mind. Not only could it play a crucial part in a war, like being forced to stop an ongoing invasion due to lack of resources, but it could also help create a more realistic support system, and an overall richer world for your story.

Conscription

Another seemingly small, but actually important detail is how your soldiers are recruited. You pretty much have 4 options: voluntary enlistment (job), forced temporary conscription, forced permanent conscription, and no conscription at all, so no army.
There are some 'sub-versions' of some of these options, or you could combine a few, but let's first take a look at how each would work.

Voluntary enlistment is the system used in roughly two-thirds of the world. Voluntary enlistment will mean everybody in your army either wants to be there or had no other choice to be there, perhaps due to a bad job market. If the majority of enlisted soldiers want to be in your army, it means they're likely more motivated, and thus training harder to be a better soldier in a shorter amount of time, resulting in a better army. However, your army size will depend fully on how many people choose to enlist, which depends on how important your army is in the eyes of society. Is it an honor to join or simply a last resort?

Forced temporary conscription means a specific group of people, perhaps all men at the age of 18 or maybe all first born children at the age of 20, will have to enlist in the army for a specific amount of time. This could be combined easily with voluntary enlistment to help grow the army, but what you're essentially creating is a population with some combat training. How much and how great that combat training is will depend mostly on how long the enlistment lasts for.
The advantage of this system, especially when combined with voluntary enlistment, is that not only will you have an army ready at all times, you will also have a big chunk of the population available to call upon when needed in times of war.

Forced permanent conscription is a more brutal way of recruiting soldiers, as it basically means anybody who was enlisted in the previous point about temporary conscription is now part of the army forever, whether they want to or not. The obvious downside is that those who do not wish to be there will be less motivated, but perhaps your army training system is able to make sure they're good soldiers anyway.
The upside is that you'll be able to recruit an army the size of whatever you wish, as long as the population can support it of course. There's no point forcing all men to enlist at the age of 18 when that leaves only half of your population to pretty much take care of everything else, that's just not realistic, at least not without slaves, animals, or another means of balancing out the forces you've now put into your army.

The last option is no enlistment. This is a bit of a weird option to pick when you're creating an army, but what it essentially means is that your country has had no need for any armed forces for the past centuries (or decades). When war does break out your country will be forced to create an army out of nowhere, and/or call upon allies to help.
While this seems like an odd option at first, it's actually a realistic option to consider. Even today there are 19 countries without any or much of an armed force at all. At the same time it can provide a whole range of drama and tension for your story.

Research

I will leave you with a final point, which by now may sound like I'm hammering it in, but do your research. Besides making sure your army, its structure, the weapons and armor they use, the tactics they use, and pretty much everything else related to your army is realistic, it will also help with inspiration. There's an enormous range of armies throughout history, some might provide inspiration for tactics, some for a certain look for your armor, and some for names for your ranks and/or units. Either way there's a wealth of knowledge available for you to explore, so why not take advantage of this.