The easiest way to do this is to find a mod using the models you like (or similar models) - the use the unit definitions they do.
Generally speaking, you need to define the unit (most people do this in XML for simplicity, although many of JFD's units use SQL - which can be greatly streamlined if you're adding multiple units in the same file). These definitions are basically database updates that add the elements the game engine needs to properly understand the unit.
You'll also need what most modders call an, "art defines" file. These are the instructions to the game engine on how to employ the 3D models.
Many of my mods use Snafusmith's mods, so they may be useful to get you started:
https://forums.civfanatics.com/resources/smans-the-world-at-war.27142/
One more thing to add: if some of the units look like they're firmly planted nose-down into the ground, it means you'll need to also use the modified animation files. Many of Snafusmith's, Wolfog's, and Danrell's models require these files as well. I can't find the direct download to these animation files, but all of them (plus modified animation files from other modders) are located in that World at War mod:
\\ART\Common\
IIRC, most of Smitty's models use the files in the 'EditedAnimations - AIM' folder, but be sure to include any modified trigger files as well. You will be able to know what files to include by looking at the model's .fxsxml file (always included in the model). This file specifies which animation files are required (ending with a *.gr2).
The modified animation files always have '_aim_' somewhere in their name. There are two folders in the WAW mod ART\Common folder - AIM and MS. I think the MS files generally deal with new ships, while the AIM files deal with everything else. But that's not 100.0000% correct all the time. It's why I included all of the files in the mod - just to be sure they were there.
The model's .fxsxml file also defines the specific trigger file the model requires (instructs the game engine how to perform animations associate with the unit). Generally speaking, a reference to a trigger file in the .fxsxml file will look like:
FX_Triggers_Fighter.ftsxml
That's standard Civ5 notation. Modified trigger files look a little different, with additional characters to denote a mod, modder, etc.:
edited_fx_triggers_Katyusha.ftsxml
fx_triggers_WAW_CW_predator.ftsxml
fx_triggers_M3A1HT.ftsxml
etc. The last one is obviously a new (non Civ5 standard) trigger file, since there's no M3A1HT unit in the game.
I know it may sound complicated, but it's really not. Just find a mod that uses a model you like, then grab the file that defines the database changes and the file that defines the animation data required. Sometimes, if you're lucky, they're in the same file - which greatly simplifies you mod's code and mod maintenance.
I only include the lengthy discussion on the modified animation files so you'd be on the lookout for them, and know how to add them to the mod if you're units don't appear, the mod crashed the game, etc. It takes almost no effort to crash the game when dealing with unit animations. So, don't be too worried if you run into a few until you get the hang of it.
One more thing to add - you may need to clear the game cache more often than usual when adding units. For example, if you're building a unit/mod several times, with a few changes, sometimes the game doesn't quite get all the changes and you'll be testing version B of your mod (in your mind) while the game is serving up version A. It often helps to exit the game entirely between builds as well. For example, if your model has a Knight with red trim and you want to change it to blue - no problem. But if you don't force the game to reload the textures in memory (i.e. exit the game then come back in), the game will use the buffered version of the file, so the Knight remains red until restart.
Lots of little ankle biters involved with adding units to a mod, but again, after you build a few, you'll amass a library of working unit prototypes that you can quickly adapt to new mods and models. Generally speaking, once you have a working Bomber model, it should be a straightforward task to add the next model using the same DATABASE code. Handling the Art Defines portion takes a little more art and finesse.
But still, hang in there and keep rebuilding through the crashes. When you're done, you'll have a mod that remakes the game in a way you think best. And that's a pretty good feeling, especially as it will introduce you to a huge community of players who really will surprise you with the quality of suggestions/complaints/etc. - and they're more than willing to work with you to make your mod and vision even better...
And when you hit a brick wall, the community here is pretty dang good at offering solutions to cut your solution time from days/weeks to minutes/hours. Basically, any problem you encounter, someone here has already encountered it and knows how to fix it.
Good luck!
