You cut the traces between the solder points at JP2 and JP3. Solder some wires. The wire(s) on the left side go to the middle of each switch - JP2 for the language switch and JP3 for the 50/60hz switch. The wires on the right for JP1 and JP2 go on either side of the language switch. The wires on the right for JP3 and JP4 go on either side of the 50/60hz switch.
This is a pretty good place for the switches on models that don't have the EXT. port. In this picture you can also see that I lifted VDP pin 50 to reduce/remove jailbars (SEGA 315-5313A).
I also cut the cartridge slot to fit Japanese games. Not that I ever use any. I only have one just to check it fits.
...and eventually you end up with:
