I wanted to make a simple Der Riese mod where I just change a few things, but I don't know how. I look up how to do so and I found and old post that said I need to use "mod.ff and IWD" It also said "Copy the factory GSC from root/maps or by decompiling the fast file(FF)." I got the Factory GSC but I don't know what to do with it exactly.

I think your talking bo3 modtools, the giant. you can port it to waw but you need to do a whole lot of text commenting and truly understand mechanics of the engine,  but it's not commenting I want you to do just use what you understand on it. And how it works in cod5 atleast it's a reply.... put it it gentle.