All scripts presented on this page require CLEO library. CLEO 2.0 for GTA3 and VC, CLEO 4 for GTA SA and the latest version of CLEO for Android.
Besides that, for most PC scripts, GInput is required:
So, even if you don’t have a gamepad, you need to install GInput anyway to use these scripts.
To install scripts simple put .cs file in CLEO folder. If there's a zip archive on release page, then the script requires additional files, copy the content of that archive to GTA root directory.