This is to save you quite a bit of money and headache in the long run.
If the PCB that you are looking to hack does not look exactly like that, IE there are the peach/brown colored spots on the board around the Left, Right, Up, Down or around the B,A,X,Y buttons, and there are no copper ovals towards the center of the board.
The Link is in the edit, SRK doesn’t allow the full link for some reason.
You CANNOT use that diagram.
Well actually you can, you just have to do the opposite, I.E. Up,Down,Left,Right, you will scrape off the opposite end. Same with the A,B,X,Y and start buttons.
If you wire it according to that diagram, you will be wiring all the grounds together and it will not work, you must wire the opposite side.
I will post a modified picture of what to do if you do not have that board.
It seems to be a rarerboard, I have had 2 of the brown kind, one was a sega sports edition and the other was an old official white.