If you tell me you're running (PC, Mobile, or Definitive Edition), I can provide more specific troubleshooting steps or keybindings.
: Use left and right turn signals for turns and lane changes.
: Locate the gta-san-andreas-real-indicator-light-mod-by-fully-update-games.zip via the official Google Drive link provided by the creator.
: Open the .zip file and move the .cs (CLEO script) files into your game's CLEO folder.
: Always back up your game files before adding new scripts.