Building a Military Tutorial

So you've set up your national abstract, you've tallied up all your provinces, assigned all your DPs, set all your policies, and you have a healthy budget surplus. Now it's time to set up your military. Depending on what your country is like, you could build anything from a small token militia force all the way up to a huge highly-trained well-equipped army of elite shock troopers, or anything in between (or, indeed, the other way around - from a small army of highly-trained special forces, to a large mob of untrained peasants with rusty old guns. Anything's possible).
=Getting Started=
 
 The first thing you'll want to do is to figure out exactly how many troops you can actually field. To do that, go to the first tab of your national abstract and look at these two fields here. The first one is how much of your population is in the military. This isn't an absolute number, as policies such as conscription can modify your net troop count by a lot. However, this will give you a baseline for your army size, and can be an indication of relative army strength. Most countries in PWP have between 2 and 4% of their population in the military. Do note that putting exactly 10% in this field will break the calculations, though any other value is acceptable. (If you want 10% exactly, 9.999% or 10.0001% will still work.) Set the number to something that feels right for now, you can always change or tweak it later.
Now the second field: Tail-to-Tooth Ratio. Put simply, this is how many of your troops work in logistics compared to how many serve on the front line. A higher tail-to-tooth ratio means your troops will be better supplied, but your front-line forces will be smaller. A lower ratio gives you a larger fighting force, but one which will be much less well-supplied, which will hurt your effectiveness on the battlefield. Modern armies, and most PWP countries, tend to have a 3:1 ratio, but you can go anywhere from 1:1 to 4:1. It's up to you.
=Designing Your Units=
You should now have a starting number of front-line troops. You might think this means you get to build your army in the Military Manager tab now, but that's not what you're going to need to do yet. First you'll need to go into the Unit Creator to make some military units that you'll then use to build your actual military. You can be as detailed as you want here; some people fill up their available slots rightaway with a plethora of units in different roles, but this is absolutely not required, and there's nothing wrong with making a minimal number of catch-all military units, if this is not your thing.
The top part of the Unit Creator contains basic information about what all the different tables mean, but I'll be summarising them again here for completeness' sake. The meat of the Unit Creator is the three tables, for Army, Air Force, and Navy, which outline your land troop designs, air unit designs, and navy designs respectively. You won't have to go into detail about your soldiers' uniform colours or the horsepower of your aircraft engines, this is mostly abstracted through the drop-down options - though it is recommended to put at least a little thought towards things like armament, and to record this information somewhere. Even if there is no option for them in the drop-down, specialised equipment like anti-tank or anti-air weapons will make a difference in an actual conflict, and any given unit's loadout will be fixed once assigned, so you'll want to make sure you don't end up fighting a huge tank army without any actual anti-tank weapons. This doesn't have to be overly complicated, just make a note somewhere that this or that unit is an anti-tank formation, or that it is equipped with anti-tank guns, that sort of thing. More detail is appreciated, but not required.
 Each of the three branches of your military has four base unit types available, under the top drop-down. For the Army, these are Light Troops, which is simply infantry with hand-held weapons; Heavy Troops, which includes things like artillery and other non-portable heavy weapons; Light Vehicles, which represents motorcycles, trucks, half-tracks, and similar units, and which can be used to transport infantry; and Heavy Vehicles, which are your tanks, tank destroyers, armoured artillery, and so on. The Air Force has Light Aircraft, which are your fighters; Heavy Aircraft, which are your bombers; Transport Aircraft, which should be obvious; and Airships, the giants of the sky. In the Navy, you'll find Light Ships, which are smaller warships, which may even be converted civilian ships, like patrol boats, torpedo boats, and minesweepers, up to things like frigates and destroyers; Heavy Ships, which are your purpose-built warships, mostly your light and heavy cruisers and other ships of similar size and capability; Transport Ships, which carry troops and supplies across the ocean; and Capital Ships, which are your battlecruisers, battleships, and aircraft carriers.
Below this you'll find drop-down options for Equipment, Training, Officer Ratio, Maintenance, and Resupply. These should be fairly self-explanatory, and units with higher-tier options will fight better than similar units with lower-tier ones, but they will also be significantly more expensive. Maintenance and Resupply are a little bit special: if you want to use the third or fourth option, you will need to have a tail-to-tooth ratio of at least 3:1, or they will be unavailable. This is why most countries in PWP pick the 3:1 ratio. Finally, you'll see two green (i.e. editable) boxes for Crew Required, and Manpower/Pilots & Crew/Seamen. Crew Required determines how many crew members are required to operate a single one of the unit's vehicles (for Light and Heavy Troops, this will always be 0); see Example Military Units for more information on which units require how much crew. The final option must be a multiple of Crew Required, and determines how many troops this unit has. For infantry, this is simply the number of infantrymen, for vehicles, planes, and ships this will determine how many of each this unit has. For vehicles, this will generally be on the order of a few dozen to a few hundred vehicles; aircraft generally come in groups of a few dozen; transport ships generally come a few to a few dozen at a time; and airships and naval vessels generally come alone.
This may seem overwhelming right now, but poke around with the drop-down menus a little bit to see what does what, come up with a handful of military units that you think your nation would use, give them an awesome name (or, y'know, just "Infantry" would do, but where's the fun in that), and proceed to the Military Manager tab.
=Building Your Army=
 The Military Manager is much less complicated than it might look at first glance. Perhaps the most important part of the page is this overview table in the upper-left. This shows you information about your country's military capabilities, such as your conscription law, maximum number of front-line troops, and maximum number of planes and ships. It'll also show you how much your military costs, once you've filled it in, and how fast your injured and killed soldiers will be replaced. The big "Operation Last Resort" drop-down is an emergency measure you can take if your country is in imminent danger of being destroyed, with a mod's permission. It will greatly increase your available manpower, but since this will be drawn straight from your workforce, secondary schools, and retirees, your GDP will drop sharply and your instability will shoot up. You can check to see what'll happen when you activate it, but otherwise just leave it disabled for the time being.
Now it's time to actually build your starting military. You should have at least a basic idea by now of what kind of setup you want to start with, after designing your units. Each of the three branches' tables works the same; you can put a group name at the top of the column - say, 1st Division - and below it you alternate between putting down the unit types this group contains, and how many of each unit you have. The unit type name must match one of the unit types you made in the Unit Creator, so make sure there's no typos. The spreadsheet will then calculate your manpower and vehicle numbers and upkeep cost. Make sure you don't exceed your available front-line troops, and that you can afford your military upkeep (check on the first tab) - if you can't, reduce the size of your military or go into the Unit Creator and pick some cheaper options. I can guarantee you, you won't be able to deploy all your available front-line troops with maxed-out stats, unless you want to go super duper triple bankrupt (you don't want to go super duper triple bankrupt).
 The image on the left is an example land army. It consists of a mechanised division - note the vehicle count; the 99 heavy vehicles is a bug and should be 100 - alongside an infantry division and a reserve unit. Note how the smallest of the three, the mechanised division, is by far the most expensive, because of how expensive vehicles are. The reserve division is almost one and a half times the size of either of the other two, but because the bulk of its numbers are scout and reserve units, it only has a quarter the upkeep cost of the infantry division. There's no one-size-fits-all solution here, just build a military that fits your nation, and make sure you can afford it in both money and manpower. (Though you could always increase taxes and/or % of pop in military to get more of either, but make sure you don't squeeze your civilian sector dry.) And keep in mind your country's situation; if you're a collective of poor farmers, where would you be getting top-of-the-line tanks and aircraft? If you're located in the mountains, tanks won't do you much good. A landlocked country has no way to field a navy. And so on. Just apply a little common sense, and if you need help, just ask in the Discord or on the subreddit.
Now go forth, and build a grand army that will make the world tremble at your feet! (Or a horde of starving angry peasants. Either or.)