Find the Game ID of the game you're looking to add. The game IDs can be found in ScummVM's compatibility list.
monkeyfor Monkey Island
.scummvmfile inside the game directory named by the Game ID
monkey.scummvmfor Monkey Island
Open up the file in a text editor, and add the Game ID.
echo monkey >> monkey.scummvm
(Optional) You could download prepared
.scummvmfiles from the libretro-database-scummvm/games directory.
Scan each game directory