So if you want to go ham on Russia here is a fairly simple way to go about it.
The first decision you want to make is what will be your "norm". Some people like 3 units per, some like 4, and some like 5. The only thing that this actually *means* is how easily you can incorporate new units as time goes on - especially when you play the USSR you're going to end up fairly spread out, so having 5 corps per army for example is going to make for a bunch of things falling out of radio range regardless of whether you're winning or not lol. I like to stick to 4 units per because it keeps everything nice and tidy.
In Soviet parlance the Army group (which will spawn in as an army group) was typically named a "front" which is a fairly helpful way of looking at it. So, in your brain bits, name out some fronts "the blah blah line" and so forth. The AI actually does this for you in TFH if you click on that theater button thing, and that could provide you with a rough guideline.
So, basically here is how I go about things as the USSR in a sort of hand hold way, in a typical game.
Step 1. Custom game, put every custom unit into Moskva (Moscow). From there I sort everything out starting with the Theater HQs. You have a "paint" button that will let you paint over the map (it's the color button thingue when you click on the theater unit on the map). I paint over everything in the West with STAVKA - that will let you delete the additional superfluous theater commands because I don't bother using them. If you want to have the AI run everything you'll need more theaters than this so it's helpful to not delete all of them, but if you're learning the game there's no harm in failing a few times doing it yourself. Then sort out all additional HQ type units and delete all of them because they're a giant mess. Then open up that little plus sign on the top right of the main map-mode screen - that'll give you a list of all the units you have, and move all your air units to Moscow too so they can join in on the Moskva night-life.
Step 2.
So now with these 3 theater HQs, I put them over broad geographic areas. Far East in the Far east, STAVKA over the enter Western front, and Stalingrad over everything in the South (Turkey/Persia/Afghanistan/Sinkiang border).
Step 3.
From there I sort out all of the units by type. So the reserve inf in one big pile, the regular inf divs in another big pile, the various light tank divs, cavalry, the 4 GAR divisions, and the 3 detached infantry brigades (I combine them into a regular infantry division because I don't have time to mess around with individual brigades). Then I ship 25 reserve infantry divisions out to the far east, along with 1 garrison division, and 1 Mountain division (for flavor) to Vladivostok. I position them all along the Trans-Siberian RR (The yellow band of infrastructure), and 1 division on Toyohara Island.
Step 4.
From there the next step is fairly simple. I just drag and drop over divisions, hit the button the creates the next level up of the command structure (f you drag and drop over several units it's the second option on the top left) and try to keep them in command range. Unless something crazy happens, you won't ever really use this command structure - it's just a place holder. So it doesn't matter what sort of wonky set up you have, just so long as you get all the units under corps command utilizing as few corps as possible. Once you've got everybody under corps commands, you pick a corps command and build an army by hitting the create next level command button (it's right under the commander's portrait or the second button on the top left if you drag and drop) and get them into Armies. You should end up with somewhere in the ballpark of 2-3 armies in the Far East. Once you've got all your corps under army commands, and you've got everybody in range of each other (you may have to fiddle with it a bit - which is why custom game in TFH is so helpful). Then you select your army command, hit the create next level button and that gives you your Army Group (Front in Soviet parlance) which you'll attach all your armies to and also attach to the theater.
Now you've got a workable OOB for the Far East. Will it survive a Japanese surprise attack? Nope. Will it deter the Japanese from invading you at random? Yes. It will also slow them down quite a lot should they surprise attack you.
As far as what the OOB actually does: A theater commander does very little, but he gives you a small reduction to stacking penalties. In the far East, that shouldn't ever really be a concern for this OOB, so you can stick the crappiest possible general you can find out here to live a nice cushy life with a prestigious sounding position. In the West (STAVKA) this will be more of an issue - particularly once you unleash the mighty Soviet Storm on Germany, so you'll want somebody a little better out there
.
I typically put a skill 1 general with logistics wizard in charge of the Far East theater HQ, because he will ever so slightly reduce supply consumption for every unit out there. Next on down you have the Front (army group) commander. His skill level is fairly important, because he reduces supply need for every unit under him based on his skill level. So stick somebody really good on there (just remember you put them out here!). Then next down you've got your Armies. The army commander raises the Org of every unit under him based on his skill level. That's super important in the West, particularly early on in any war with Germany. But he also gives every unit under him 25% of his traits. Since these guys will never be fighting (and if they do they'll be losing fairly badly), you may as well just stick some unskilled logistics wizards out here in the Far East to further reduce the drain on your economy for having to guard this area. For Corps commanders, they won't matter at all in the far East, but they do give 50% of their traits to any unit under them, so if you find you've finished your entire OOB and you've still got some logistics wizards laying around, you can stick them out here (you shouldn't). I tend to fill my Far Eastern Corps with old guards so I don't have to look at them - same for the divisions they command (unless they have really sexy traits).
Now I'll go through another example for the USSR, 1941. The big party lol.
1. STAVKA theater command has control of the entire western front, from the Romanian border to Murmansk and the Arctic (if you want to use the AI it's suggested you not do this because the AI will march troops to Murmansk, to Odessa and back again to taunt you for making it work too hard). I put a skill 3 (or if you've leveled somebody up to 4 that'd be ducky too) Logistics Wizard here, because he's gong to be commanding some stupendous amount of units, and a super-small reduction to their supply usage over so many units adds up to a pretty big number General Meretskov for example. I put the actual Stavka unit in Moscow because reasons.
2. 4 Fronts (army groups). One to cover the Arctic/Finnish border - the Arctic Front, one to cover all of the land between Leningrad and Pinsk/Minsk - the Leningrad Front, then one to cover all of the land South of Minsk to Kiev - the Kiev Front, and lastly an army group to cover all of the land from South of Kiev to Odessa - the Odessa Front. You want your very highest skilled Generals here to reduce the, likely, tremendous supply drain on your economy. Zhukov, Konev, etc.
3. For the two main fronts (Leningrad and Kiev), I have 3 armies of Infantry, and 1 army of Mech/Mot and/or Tank divsions. For Odessa I typically have 1-2 primarily INF armies, and 1 Armor army (with some CAV added for fun and horsies) and in Finland typically 1 INF army or 2-3 depending on the status of Finland (comintern puppet/annexed - or free and Axis leaning).
4. Each Army consists of roughly 4 Corps, sometimes fewer depending on numbers. Each army is commanded by somebody with 3+ skill, with special emphasis on the armies which have my Tank and Light tank divs in them. I find that putting tanks and light tank type divisions in their own armies just makes my brain happy.
5. Infantry Corps consist of 3-5 infantry divisions who happen to be relatively close to each other. I try to include at least 1 AT carrying division and 1 division that's Inf/inf/ART/ART - but that's not essential or necessary. Corps commanders (once you research the land tech that gives you the unit cooperation bonus) basically don't matter at all as far as what their skill points give you, so I stick low skilled guys who have nice traits (defense, offense, log wizard) here.
Tank corps consist of 4-5 ARM divisions, and each tank corps has a buddy corps of MOT, CAV, or LARM divisions. Corp commanders are selected for traits (Panzer + any other trait or offense + defense). Then I just aim the whole Corps and all of their units at some area that has plains on it (the best spot to fight with armor).
Then you simply attach the three fleets to whatever Front (army group) makes you happy - so they eat less supplies. Usually for me arctic fleet to arctic front, Baltic fleet to Leningrad front, and Black Sea to Odessa front.
Then you assign air craft at your pleasure, and attach them to whatever front they vaguely fit into (again for the supply bonus) but it it's not super important that they be under the proper front command - it just makes them easier to find and know they'll be relatively close to whatever you want them to do in that area lol.
And then you've done it you have a working OOB for the Soviet Western front in WW1. I typically start building the OOB and moving everybody into their proper place around April 1940, with everybody generally fully set up and etc by March 1941.
As you fight the war you'll find stuff falling out of command range. That's inevitable and shouldn't bother you too much, ideally you'll just be able to detach and reattach them to somebody else closer by - that's why I like corps and armies to only have 3-4 divisions (or corps if we're talking about armies) so they have space to accept stuff that's gotten separated from their old friends. Also take a peek at your division, Corps, and Army commanders as the war goes on, because their skill levels tend to swell nigh uncontrollably, and you may want to promote them to take advantage of their bloated skill level at either the Army or Front (army group) command levels.
A thing to keep in mind, is that ultimately your OOB is a function of your mission:
If you're just guarding something that will only very rarely be attacked, the emphasis is to build as few HQ units as possible to save supplies.
If you're defending statically (that means you intend to hold your position and never move from it) you'll likely end up with a Corps or more worth of troops in every pertinent tile/province a la France on the Maginot line or a static defense of Belgium later. Or the "Molotov" or "Stalin Line" defenses for the USSR.
If you're defending in depth things will be *far* more spread out than when you do a static defense, and the OBB is primarily there purely for your own sanity and to keep things loosely organized.
If you're attacking it's easiest to think of things, for me, in terms of Armies. This army will take x objective place, then you attach corps to make it happen.
If you're going on the offensive after a time of defense, it doesn't matter as long as you've got some number of units in radio range to give yourself the best chance of success lol.
