Old school method of stopping the motor from turning over, instead of either having a buddy hold the clutch, or worse sticking a pipe in it and damaging it..
Remove the plug on the side of the tunnel, remove the clutch bolt, (see above) it might be a tight one.
Once that is off, you need the correct clutch puller for your sled/clutch. Once you have it, a couple pumps of greese in the opening will do wonders, put the puller in the same hole the clutch bolt came out of and start threading it in. Once it gets tight, a nice four way works to tighten it down, and you will hear a POP/BANG and the clutch will come off.
The grease helps with this by acting as a hydraulic push. The clutch is just pressed on, one a tapered shaft. Pressure keeps it there as well as takes it off