Before modding GTA SAN ANDREAS it is best to reinstall GTA to a clean version and make a copy of your game. After you do these steps it would be worth having 7-zip or winrar to unpack files with fixes and mods. here are links to 7-zip and WinRar.
7-zip[www.7-zip.org]
WinRar[www.win-rar.com]

Anyway with all of that out of the way, let’s start by downgrading the Steam version of San Andreas.

Step 1: Download TJGM's Downgrader[mega.nz].

Step 2: Launch San Andreas on Steam before you downgrade. If you don’t do a first time launch before downgrading, Steam will possibly update San Andreas and undo the downgrader.

Step 3: Open your San Andreas folder. You can find this by right clicking San Andreas in your Steam library, click “Properties…”, click “Local Files”, and then click “Browse”.

Step 4: Open the downgrader archive that you just downloaded. Copy all of the files from the archive, into your San Andreas game folder (this is where gta-sa.exe is located). Replace all of the files when asked.

And that’s it, San Andreas is now downgraded.

However here’s the thing, if Rockstar for some reason ever updated the Steam version of San Andreas again, it will revert the downgrader and if you have mods installed, it will break your mods. It’s very unlikely they’ll update it now as it’s no longer for sale on Steam, but there’s still a small possibility.

So you have two options. You can take the really small risk, and continue to launch San Andreas through Steam.

Or, you can rename your San Andreas folder so Steam will no longer detect it. The downside to this is, you won’t be able to launch the game through Steam.

It’s up to you really. If you do rename your San Andreas folder, you can delete “gta-sa.exe”, and just launch San Andreas by clicking “gta_sa.exe” instead.

There’s no difference between the two exe's other than Steam uses “gta-sa.exe” instead of “gta_sa.exe” to launch the game. Both executables are the exact same aside from their names.

If you’re launching San Andreas through Steam, I’d recommend keeping both “gta-sa.exe” and “gta_sa.exe”.

The reason for this is, some REALLY old mods may expect the games EXE to be called “gta_sa.exe”, so they won’t work if you launch the game through Steam, which uses “gta-sa.exe”.
Again, these are really old mods that you’ll likely never even see. If for some reason a mod you want does need the games EXE to be called “gta_sa.exe”, you’ll just have to launch the game through that EXE instead of through Steam.

Our next step will be to download Cleo and modloader which are very necessary for installing mods and fixes.

Step 1: Download The Modloader[www.gtagarage.com] and Cleo[cleo.li] for GTA SAN ANDREAS.

Step 2: If you have already downloaded these modifications, extract them to any folder using 7-zip or WinRar.

Step 3: Copy the files of Modloader & Cleo and replace it in the Grand Theft Auto San Andreas folder where gta_sa.exe and gta-sa.exe is located. You can find this by right clicking San Andreas in your Steam library, click “Properties…”, click “Local Files”, and then click “Browse”. That's it you installed Modloader and Cleo.

The third step will be to install the SilentPatch, which fixes game bugs such as a non-working mouse or ALT+F4 not working.

Step 1: Install the SilentPatch[cookieplmonster.github.io]

Step 2: Extract files from SilentPatch to any folder.

Step 3: Copy the files from SilentPatch folder and replace it in the Grand Theft Auto San Andreas folder where gta_sa.exe and gta-sa.exe is located. You can find this by right clicking San Andreas in your Steam library, click “Properties…”, click “Local Files”, and then click “Browse”. That's it you installed Modification that fixed your game alot.

Our fourth step will be to install the widescreen fix which fixes the resolution in the game. This modification is mandatory to install because when we play on a native 16:9 monitor the game may be stretched and in other cases even the game does not have the resolution of the monitor if you do not have this patch.

Step 1: Download the Widescreen Fix[github.com]. Remember to Download GTASA.WidescreenFix.zip and GTASA.WidescreenFrontend.zip on GitHub.

Step 2: Extract these two files to any folder using 7-zip and winRar

Step 3: Copy the files of Widescreen Fix and replace it in the Grand Theft Auto San Andreas folder where gta_sa.exe and gta-sa.exe is located. You can find this by right clicking San Andreas in your Steam library, click “Properties…”, click “Local Files”, and then click “Browse”. That's it you installed Widescreen Fix that may help you with resolution in game.

the fifth step will be to install Project2DFX which adds LOD corona effect to the game map which will make the LOD world look much better.

Step 1: Download Project2DFX[github.com].

Step 2: Extract these files to any folder using 7-zip and winRar

Step 3: Copy the files of Project2DFX and replace it in the Grand Theft Auto San Andreas folder where gta_sa.exe and gta-sa.exe is located. You can find this by right clicking San Andreas in your Steam library, click “Properties…”, click “Local Files”, and then click “Browse”. That's it you installed Project2DFX that makes game looks clean.

the sixth step will be to install RemasteredGUI which changes the texture quality in the menu to HD quality.

Step 1: Download the RemasteredGUI[www.gtagarage.com].

Step 2: Extract these files to any folder using 7-zip and WinRar

Step 3: Copy the files of RemasteredGUI and replace it in the Grand Theft Auto San Andreas folder where gta_sa.exe and gta-sa.exe is located. You can find this by right clicking San Andreas in your Steam library, click “Properties…”, click “Local Files”, and then click “Browse”. That's it you installed RemasteredGUI.

the seventh step will be to install the Map and icons in HD quality. The installation will be different from the others.

Step 1: Download the HD San Andreas Map[www.gtagarage.com] and HD Map Icons[www.mixmods.com.br]

Step 2: Unzip the folders separately.

Step 3: Copy the folders of SA Original Radar Icons HQ and hqr-sa to modloader folder in the Grand Theft Auto San Andreas folder where gta_sa.exe and gta-sa.exe is located. You can find this by right clicking San Andreas in your Steam library, click “Properties…”, click “Local Files”, and then click “Browse”. That's it you installed Widescreen Fix that may help you with resolution in game.

Our penultimate step will be to install a frame limit of 60. Why? The game has a lot of bugs if it has more than 60 frames per second and strange things can start happening above 60 fps.

Step 1: Download the Framerate Vigilante

Step 2: extract the modification fo any folder or even desktop.

Step 3: Copy the folder of FramrateVigilante to modloader folder in the Grand Theft Auto San Andreas folder where gta_sa.exe and gta-sa.exe is located. You can find this by right clicking San Andreas in your Steam library, click “Properties…”, click “Local Files”, and then click “Browse”. That's it you installed Framerate Vigilante.

Our last step will be to install the weapon icon on the hud in HD quality

Step 1: Download HD Weapon Icons[www.gtainside.com]

Step 2: Extract Only Folder Icons from folder modloader in archive that you installed using 7-zip or WinRar.

Step 3: Copy the folder Icons to the modloader folder in the Grand Theft Auto San Andreas folder where gta_sa.exe and gta-sa.exe is located. You can find this by right clicking San Andreas in your Steam library, click “Properties…”, click “Local Files”, and then click “Browse”. That's it you installed HD Weapon Icons.

That's It you fix your Grand Theft Auto San Andreas. Let me know in comments that you like this guide and i can create for more games guides like this. Enjoy The game and Have Fun.