Place the two batteries back to back, close to each other with clear access to their positive and negative terminals.
Connect the negative terminal on one battery to the positive terminal on the other using a length of battery cabling as short as possible to make the connection. With DC (direct current) electricity, you can lose power through excessively long cabling, so keep the cables as short as possible.
Set the dial on your multimeter to measure DC voltage.
Connect the red positive probe from your multimeter to the remaining open positive terminal on the battery.
Connect the black negative probe from your multimeter to the remaining open negative terminal on the battery. Your multimeter should now read an output voltage of 12 volts; wire the positive and negative terminals you just measured to the 12-volt device you want to power.