LED Control Board Installation Guide
1. The LED board
If your board has bridge pads, solder a wire to each pad.
If your board does not have bridge pads, solder one wire to the LED board and another wire to the pad on the game boy. Make sure you insulate well.
Feed the wires through the battery bay and solder the two wires to the LED control board
You can place the board where ever you chose. Here is how I place it, which involves zero shell trimming (except for the GBA)