You probably won't need this often as most games are mono anyway and games which are stereo usually have a mono option (for the JAMMA standard) but if you have a multi-slot Neo Geo motherboard here's how to stop it from sounding a bit one sided:
"Standard" Formula for converting Stereo to Mono:
Mono = L/2 + R/2 = (L+R)/2
Dividing a signal's amplitude by 1/2 is simple -- just use a voltage divider formed with 2 equal resistors. The "adder" in your case can be just hooking the wires together.
Okay... That's the textbook answer, here's what it means! Bridge the + and - speaker wires with a 4ohm resistor and use 2ohm resistors instead of wires to
connect the +ve signals. This will keep it all at 4 ohms and roughly the same volume. Add capacitors to taste for 1st order crossover (not THAT necessary).
The optional JAMMA fingerboard is to make things easier to connect, after all you don't want to have to disconnect wires inside your cab/supergun to perform the mod!!! Also, if you're making a multislot Neo Geo loom you'll also need to make the correct connections for the test button and the game select buttons as well as its NOT JAMMA like the one slots are.