1] Use FinalBig to either edit an existing .big file or create your own. For our purposes I edited the Misc.big file (make a backup first) and removed all files except aidata.ini, skirmishaidata.ini, and gamedata.ini. My purpose: to increase game speed 3x faster and give 75% cost back when selling buildings. Game speed increase, though, is real fun and will allow VERY FAST Skirmish AI testing.
2] Now once you have your custom .big rename it to something else like Misc-Custom.big. Ensure you have your unedited Misc.big or whatever original big file you used to make your custom one.
3] Where your big file is inside C&C3's folders look for a "config.txt". For this example, since I made a custom Misc.big file and called it Misc-Custom.big this is what my config.txt looks like:
add-big StaticStream.big add-big StaticStreamL.big add-big GlobalStream.big add-big Art.big add-big Libraries.big add-big Shaders.big add-big Apt.big add-big Misc-Custom.big add-big Misc.big add-big WBData.big
Note the position of your custom big file in relation to the original big it is overwriting. Yep.. it needs to come first! This will tell the game at runtime to use those custom files inside your own custom .big file (Misc-Custom.big) instead of the one in the untouched original .big (Misc.big).
4] Run the game and now your game is customized without altering the existing original .big files.