There was a request for a step by step how to create a selectable mod and add a new unit. Apperently all the information is out there but no single spot to get it. I will take on the task. Additions or corrections are appreciated.
First off in you supcom folder make a mods folder.
Inside that, make another folder that is named for your mod.
Inside that, make a "mod_info.lua" file.
that file should look like this
author = "Your Name"
name = "Your Mod's Name"
copyright = "Copyright © 2007, Your Company"
version = 1
description = "What does this mod do."
exclusive = false
ui_only = false
Now. make a "units" folder.
Inside that, make a folder named for you units ID, you can follow the naming scheme of gpg, or just make up your own ID.
Inside that, Coppy all the files from the unit that you want to start from (or start from scratch if you want, But I don't recomment it for a beginner.)
Rename all the files so the ID portion of the name matches your units ID.
Inside the units _scrip.lua file you will see something like
URL0104 = Class(TLandUnit)
change that so the URL0104 part matches your units ID.
At the bottom of the file there is
TypeClass = URL0104
again change that to match your units ID.
Inside the .bp file there are some things you can change to help you identify your unit in game.
Description = '<LOC uel0104_desc>Mobile Anti-Air Gun',
UnitName = '<LOC uel0104_name>Archer',
HelpText = '<LOC uel0104_help>Mobile Anti-Air Gun',
Now, create a supcom shortcut and open its properties and append this to the end of the target. (outside the quotation marks)
/windowed 1024 768 /EnableDiskWatch /nosound
That will as it looks, start the game in a window (so you can see the log), enable the game to keep an eye for changed files so that you don't have to restart the game every time you make a change, and turn off sound(its kind of annoying while testing).
Using that link start the game, go to skirmish mode, choose
Fog of War = None <-- so you can see the whole map(good if you are testing artillery)
Victory Condition = Sand Box <-- game dosn't end
Game Speed = Adjustable <-- sucks waiting around while things move or build
Cheats = On <-- so you can use debugging tools
Ok, start the skirmish.
Now, hit F9 to open the log and look for any yellow text as these are warnings. You shouldn't see any. Unchecking Log and Spew should help with this.
Now, click on the map somewhere and hit Alt+F2.
Find your unit in the list and click on it.
Also, your unit should be buildable from the same place as the unit you started with if you didn't change any built by stuff.
Again, check the log for errors.
After that, tweak to your hearts content.
Note, even though the enable disk watch is on it is kind of flakey in my experience. If you think something should work try exiting back to the main menu and restarting the skirmish. Also, making changes won't affect units already placed so select them and Ctrl+Delete to get rid of the old version and place a new one.