The jumpers you need are on the underside of the board near the joypad ports. So turn the board over and position it so that the joypad ports are to your right.
Now look around by the top joypad port, just above the thick white line that goes through the centre of the board.

You should find the cluster of jumpers (6/7,8/9,10/11,12/13).
Once you've found them you need to break the existing connections that are there.

Assuming you've got a European Saturn the jumpers will be connected like this

Once you have removed the links you need to solder 3 wires, one on to each jumper pair. You need to solder onto one of the 'common' pads in the jumper pair. These common pads are indicated in red in the previous diagram.