First you need clead Steam version of San Andreas, and disable autoupdate (Right click on San Andreas on Steam > Properties > Updates)
Also you need to know where your San Andreas folder is (Right click on San Andreas on Steam > Properties > Local files > Browse local files...)

Your San Andreas version must be downgraded to version 1.0 google how to do it, there are different ways to do so. Im not going to explain it there, sorry.

Install San Andreas Multiplayer[www.sa-mp.com] to your San Andreas folder

You can skip this if you are not playing SA-MP

Installing Multi Theft Auto[www.mtasa.com] stop at "Choose Install Location", Destination Folder choose "Custom" and click on "Browse..." browse for your "Grand Theft Auto San Andreas" folder, click on "Make New Folder" name it "MTA San Andreas" hit "Ok". Click next and install

You can skip this if you are not playing MTA

Dowload San Andreas Launcher[www.dropbox.com], and put "gta-sa.exe" to your San Andreas folder. Now when launching San Andreas from Steam you can choose what you want to play Single Player, SA-MP or MTA.

With this launcher its lets you choose what you want to paly singleplayer, SA-MP or MTA.

This launcher is made with AutoRun Pro.

You can install any mods you like, they will work


Want to play in 1080p
Download ASI Loader[www.gtagarage.com] and copy all files to your San Andreas folder
Download sa_widescreenfix_lite[www.gtagarage.com] and copy "sa_widescreenfix_lite.asi" to Your San Andreas folder

If you used "How to play San Andreas in true 1080p" guide. Skip my guide to part "Install SA-MP and MTA" and rename your "gta-sa.exe" back to "gta_sa.exe".