Step 1: You need to have GTA San Andreas installed on your PC
Step 2: Install modifications on your PC usin SAMI
You can see that I have added a photo of that SAMI. To download this you can google it and easily find. After that, when you open it, you can see clearly an option as "Install Mod". Click on it and then select the modifications which you would love to add. As far as they are concerned, there are plenty of sites on web who provide with different modifications of GTA San Andreas game.
In other image, you can see how you will get various options. You will be easily guided. Don't worry and panic at all that you game file will be corrupted. For this, I have given a precaution to backup the main game file.
After all this is done, you are now in a position to start your modified Grand Theft Auto San Andreas video game.