Download the mod file from a trusted source
Locate the BeamNG.drive mods folder
Open the mods folder
Copy the mod file into the mods folder
If the mod is a .zip file, keep it zipped
If the mod is a .pc file, place it in the correct folder if required by the mod author
Launch BeamNG.drive
Open the Mod Manager
Enable the installed mod
Restart the game if prompted
Verify the mod appears and works in-game
Remove or disable any conflicting mods if needed