Open Crazy IMG Editor.
Locate the gta3.img file in your GTA game directory.
Open gta3.img using Crazy IMG Editor.
Import the mod files into gta3.img (these are usually in .dff and .txd formats).
Use the import function in Crazy IMG Editor to add the model files to gta3.img.
Close the crazy img.
Start GTA San andreas to see if it works or not, if not then, go watch youtube video how to install mods in gta3.img using crazy img.