You can mod GTA 5 in two ways: With .ASI mods and with .DLL mods. Either way, you'll need to install some other add-ons like Script Hook first.

Installing Mods in GTA 5 Here’s how to install mods in GTA 5: Download and install Script Hook V. Extract the files and move ScriptHookV.dll and dinput8.dll to the main GTA 5 folder.