There are different types of tire groovers. The one I used I bought it from SpeedwayMotors.com
I also bought a few dozen 1/4" cutting blades to go with it (only needed 2 though for all 4 tires).
Basically just let the iron heat up for about 15 minutes and then begin grooving. I sat on a little stool with wheels so I could roll the tire accross the garage floor as I'd cut it.
Slow and light pressure is all it takes. If pushing too hard and fast your arm will get really tired/sore.
attached is a blown up photo of my avatar showing the pattern I used when I grooved my TSL/SX tires. Your pattern would be different of course but the principle is the same. The grooves give additional biting edges to grab onto things like rocks and allows the tires to not be so stiff and helps them to flex/conform to the rocks better
and here is a link to some other peoples ideas they used for tire grooving... some of them are the boggers