First, you need to download the
IMG Tool 2.0. Put the IMGTool.exe file in your main San Andreas directory and then open it. First off everything, remember to back up your gta3.img file, that you'll edit later, located in the models folder. Now, at top left you'll see a button called
File. Click on it and then click
Open. Now search in your San Andreas' model folder a file called gta3.img. Open that, and there's you'll see all kind of files, most of them ending with .dff. Now, press
Commands in the IMG Tool and then
Add. Now search for the files of the mod you've downloaded and select them. That should be it.
If you have any trouble doing this, tell me.