The Game Genie for the Super Nintendo is a gaming peripheral developed by Galoob in 1992 that enabled gamers to apply cheat codes to their favorite SNES games. It gave players the opportunity to have infinite lives, unlimited ammunition, invincibility and many more attributes to help win any SNES game. The Game Genie functioned by joining together with a Super Nintendo game, and then loaded into the SNES system. From there, a sequence of digits are inputted to use codes that manipulated the parameters of an SNES video game.
Insert an SNES game in the slot on top of the Game Genie. Insert the Game Genie into the cartridge dock of the SNES system. Ensure that the toggle switch in front is set to the upper position.
Turn on your SNES system and wait for the green light on the Game Genie to appear. Once the green light shows the Game Genie menu will appear with a series of numbers from 0 through 9 and letters A through F.
Navigate to a each letter or number using the directional pad. Press the "A" button to enter the a letter or number into the code line. Press the "B" button to erase the last letter or number entered.
Continue Step 3 until you have entered a code that is eight digits long. You can input a total of 5 lines of game codes. When you are finished, press "Start" to begin your game with the desired cheats activated.
If the green light does not appear, that means the toggle switch is set down and you will start your game automatically without going into the Game Genie code screen.
(taken from https://itstillworks.com/12535503/how-to-turn-on-god-mode-in-gmod)