Okay, so I know others have done this. But for some reason, I can't find a tutorial on how to do it, after multiple searches.

I want to just make a custom terrain paint into a default replacement neighborhood terrain- I'd like to do one for the Lush terrain and the desert terrain. If someone could point me in the right direction, I'd appreciate it.

You need to extract textures from the game files and put them in a separate package. For the lush terrain you can use files from my default replacement (just replace the textures with yours), and based on the image names you can see there, find proper files in the base game folder\TSData\Res\Sims3D\Textures.package for the desert version.

I was hoping for a more detailed walkthrough than that, but I'll give it a guy. I guess I could use anyone's default desert terrain files as a base and replace them. It sounds easy enough, but I'm always afraid I'll muck something up. Guess I'll give it a go.

I think you’ll manage just fine As long as you don’t edit the game files themselves nothing bad should happen.

You're right. It's much like default replacement a skintone. Almost exactly, in fact. Thanks for pointing me in the right direction.