For this tutorial, we are going to use a skin mod as an example. Any kind of mod that replaces textures can be installed the same way.
Step one:
Make sure that you have version 1.0 of GTA San Andreas. If you're unsure of which version you have, then run the downgrader just in case you have version 2. Modifications are only supported by version 1.0. The downgrader patch can be found here.
Step two:
Download a suitable IMG editor. The game files of GTA San Andreas are in the format .img and thus, you will need an IMG Editor to edit them. I personally suggest you to use the Alci's IMG Editor which can be found here. It is one of the best IMG editors out there and is a modified version of Spark.
Step three:
Backup your gta3.img. To install a skin mod, you need to place the skin files in the gta3.img thus, replacing the original files. In case you want the modification to get removed, the backup might come in handy. If you don't want to make a backup, then you can find the backup of individual files and the whole gta3.img here.
Step four:
Choose between gta3.img and custom.img. If you want the modifications to show up only in SA-MP, then you need to place the mods in the custom.img file which can be found in GTA San Andreas>SAMP. If you want them to show up in both singleplayer and SA-MP, then you have to place the files in the gta3.img found in GTA San Andreas>models.
Step five:
Download your skin mod. It should consist of two files with the extensions, .dff and .txd. If you have downloaded other files, then it is a modification for CJ for the singleplayer version of the game. To download a skin mod, I would like to recommend these sites to you :
Step six:
Open up your .img files using the IMG Editor. Import the skin files using the import button and when asked to replace, make sure you select "yes". Now save it and rebuild it with the respective buttons.
Thats it!
You have successfully installed a skin modification. If you have any further inquiries, feel free to submit them here.