A locking tab on the bottom of the shell (under the base) holds the base in place.

Stick a long, skinny flathead screwdriver through one of the vent holes on the back of your Xbox, near the locking tab.

Insert the screwdriver between the locking tab and the base. This might take a few tries—use a flashlight if you're having trouble.

Use the screwdriver to pry the tab inward (away from the base) into its open and unlocked position.

Keep the tab held open for the next step.