Have you studied the grblShield wiki?https://www.synthetos.com/wiki/index.php?title=Projects:grblShieldIt looks like grbl takes over the ATmega processor in your Arduino UNO and doesn't use the Arduino bootloader so it can't be uploaded as a sketch. You will need AVR Studio 4 and a ISP programmer to load the grbl .hex file into your ATmega328P.
Also how should I wire my nema23 motors into the grbl shield. I'm sure it is not as simple as just connecting them to the shield and slotting the shield onto the arduino. How should I wire up an extra power supply and resistors?