Well the basic schematic is 12V supply to relevant screw-terminals on the shield, the two motors to their relevant screw terminals on the shield, plug shield into Arduino (which will need power somehow - this could be derived from the 12V motor supply but that can present issues if that is too noisy or overloaded by the motors). Decoupling for the motor supply is probably the cylindrical electrolytic on the shield, so that all looks good.
Ensure the 12V supply polarity to the shield is correct(!)
Those motors will need PWM drive, ramping up from 0 (don't directly set to 255, that will strain the supply and be very rough on the gears). I've got a similar (but 600rpm) gear motor and it runs very smoothly at low drive levels...
Have a search around these forums for LM298 examples. When first testing I recommend using a weakish power supply for the motors (perhaps 4 x AA's might do), check all is well before moving to the proper supply - a little paranoia at the right time can prevent silly mistakes.