"Just the place for a Snark!"" the Bellman cried,/ As he landed his crew with care;/ Supporting each man on the top of the tide/ By a finger entwined in his hair.// "Just the place for a Snark! I have said it twice:/ That alone should encourage the crew./ Just the place for a Snark! I have said it thrice:/ What I tell you three times is true."//
OKaaaaay. A TC is a big commitment, but lets assume you're in it for the long haul.
1) An introduction to filetypes:
- .map (or similar)
Mixes are the archives which contain all the files used by the game. You can open, extract from them, and edit them using the XCC Utilities (Xhp.xwis.net). There are various rules about what goes into which mix, but I'll get to those later.
Shps, "shape files", are the 2D stuff in TS. That's buildings, infantry, GUI, etc. They're composed of a set of frames which are bundled into .shp files. They can be edited with SHP Builder (Ppmsite.com). Buildings and infantry are both easiest to produce in 3D, then be rendered down to TS perspective. There is a script for this, but it's oooooold. DonutArnold's template is much more useful (Ppmsite.com).
Vxls, "voxels", are the 3D-ish stuff in TS - the vehicles. They can tilt and rock, and are a reasonable approximation of 3D models, at a much reduced resolution. They are possibly the most straight-forward graphics to create once you have an understanding of the Voxel Section Editor (Ppmsite.com).
Tmps are terrain tiles. They're all the ground in TS. The format is also used for some of the embelishment stuff like trees (but they're edited as SHPs). Terrain is assembled with the TMP Builder (iirc, included in the XCC tools) or TMP Studio (Ti.ppmsite.com). Obviously it's easiest to actually produce the terrain in another program like Photoshop, GIMP, 3dsMax or the like.
INI files are the config stuff for TS. The most important ones are probably rules.ini (all the unit stats and such) and art.ini (frequently invoked by rules.ini, and unsuprisingly containing all the stats for artwork). They're editable in notepad. There are plenty of hardcoded things a weirdness though.
Maps are... maps. They're pretty straightforward as things go, and are editable in the venerable (old and buggy :/) FinalSun (Commandandconquer.filefront.com). Maps can easily be broken by alteration to terrain (and the inis governing terrain).
There are a few other file types which I'm ignoring here for simplicity's sake. .vpl, .pal, .vqa, and a few others which are of niche interest. And there's .aud, which is sounds. I know very little about them.
As for starting at a certain area, a good understanding of TS' ini format and its conventions is probably required. Having team members to cover 3D and 2D work certainly helps if you want to do any visual alterations. have an explore of the mixes and look at how TS is laid out on that level.
I would suggest starting by trying to get a building, an infantry, an SHP vehicle and a voxel vehicle fully functional and in game. Then the rest of the project is just iterating on that process with creativity, and then balancing the results. Also, experiment first, ask questions later.
,.. I should probably have a step 2). Well, once you've got the filetypes down I can walk you through how to actually make vxls and shps. Code is slightly outside my purview, and I'm a bit shaky on the specifics there.
So yeah... Good luck. You will need it.