The GBA has a very awkward button layout when used with LSDj. This mod converts the trigger buttons to 'start' and 'select' allowing for comfortable use with LSDj. Keep in mind that you will need to add a switch if you would like the buttons to work as per normal again.
You'll need to note the pads:
TP2 = Select signal
TP3 = Start signal
The triggers signals unfortunately don't have pads like this, as they go pretty much directly into a resistor. (R43 and R44)
It's as simple as soldering pad TP2 to a trigger signal, and TP3 to the other signal, then just remove the two SMT resistors (R43 and R44). It doesn't matter which one you solder to, it all depends which button you want to be 'start' and which button you want to be 'select'
Use extremely thin gauge wire, these pads will probably rip out with anything thick.
Here's where I soldered from pad TP3. I put the wire through the little cutout in the circuit board. It shows the removed 'R43' resistor. Removing this basically stops the trigger from changing the aspect ratio to 16:9.
Here's where I soldered from pad "TP2". I put this wire through the top of the tact switch then through the cut out, to avoid interference with the d-pad contact pads.
And that's it! If you want to reconnect these wires, you just need to restore the connection. I do this by using a thru-hole resistor to replace the SMT ones that were removed and toggle between the connections.