[Extremo]

Hey,

Get yourself IMGTool(since a lot of the skins do not have a pre-installer with them).
Take a backup of gta3.img file somewhere in your GTA SA Directory.
Open up IMGTool and open the gta3.img file within your GTA SA Directory(not the backup!)
Drag the files, .txd and .tff if I remember correctly into your IMGTool and replace them with any existing files.
Save the archive.
Enjoy.

Incase things go wrong, replace gta3.img with the backup you took.

Regards.

PS: SA-MP does not advise you to use any mods, as they may make the game unstable and so forth. If you however mod your GTA, it is at your own risk.