Step 1: Have the original version of GTA 5 (Steam, Epic Games or Rockstar Games Launcher).
Step 2: Download the client by visiting FiveM's official website: https://fivem.net
Step 3: During installation, GTA 5 will automatically detect the file location. If not, enter it manually.
Step 4: After FiveM is installed, run it from the desktop and connect to the server you want.