Step 1: Stuff you Need
So, you need:
Two LEDs (Color of your choice)
Fine Sandpaper (400 - 500 grit)
Tri-Wing Nintendo Screwdriver or small flathead screwdriver
Transparent Purple Gameboy Color
Helping hands (Optional, but really helpful)
Hot glue gun and glue (Optional)
Step 2: Take it Apart
Lift the bottom half of the case off the top one, then unscrew the three screws near the lower half of the PCB. Make sure you save these.
Once you have that done, you need to disconnect the screen. Lift both black plastic pegs toward the orange ribbon cable at the same time, then lift the cable from the connector.
Step 3: Cut the Switch Hole
Start by scoring the case of the Gameboy in the spot you want to put the switch. I found there is a lot of room just underneath the battery compartment. Cut the hole a little smaller than needed. You can always cut it bigger, but you can't cut it smaller. Keep cutting until you cut all the way through. It takes a long time, but it's worth it for a clean cut.
After you have the hole cut out and the switch fits, drill the holes for the screws. I like to put the switch in first, then drill through the hole for the screw.