Alright. For this, you are going to need San Andreas Mod Installer. Again I'm too lazy to provide a link to the DL.
Ok so the mod I'm going to install is a Chevy Suburban to replace the FBI Rancher. Whence SAMI is done, open it up. Then press "Install A Mod"
Then press the button "Detect" to make sure it knows where your game is. Then you want to change it from ZIP/RAR file to Folder. Then press browse and look for the folder of the mod you downloaded. Press Continue twice.
Now give the mod a name to identify it if you want to uninstall it. As for me FBI Rancher will already be selected, and I will give it the name "Chevy Suburban FBI". Then press Install, and have fun with it!