copy the crack, and click paste in the directory with the SA .exe file.
Example:
Install directory for San andreas = C:\Games\GTA SA
Wherever ur crack is, copy it and paste it into the above folder overwriting the original. You may want to backup the original .exe, as i always do. Just keeps it somewhere incase it needs old files to update.
I believe the .exe should be called 'gta_sa.exe'
really easy thing to do