To install GTA San Andreas Mod APK on your Android device, first, enable "Unknown Sources" from Settings > Security to allow third-party app installations. Next, download the Mod APK and OBB files from a trusted source. After downloading, use a file manager to extract the OBB file and move it to Android/OBB/com.rockstargames.gtasa. Then, install the APK file and grant the necessary permissions. Once the installation is complete, launch the game and enjoy the modified version of GTA San Andreas. If you face any issues, restart your device or check if the files are placed correctly.