Flip the GameCube upside down.

Use a 4.5 mm gambit screwdriver to unscrew the four screws located on each corner (as shown in the second picture).

Remember to put all screws aside and out of the way from risk of being lost

With the four screws removed, flip the Gamecube back over to the original position.

Make sure to hold the bottom wall so it does not come off when flipping back over.