Bruce has a how to video... I think it has a thread in his section here.
It's not hard to do. Here is a pair I cut recently (one for a mini MAP tank and the other a regular)
I wrap a rubberband around the end that comes off after cutting to get better grip while spinning it. If you've never used a pipe cutter... you line it up, tighten it up and spin the CE2. After going all the way around a time or 3, tighten a little more and spin again a couple times. Repeat until you hear split and continue to spin until the extra piece comes loose. Another tip... once cut, use something to push the piece shown in my image out. If you just pull them apart, the cup may come loos from the center air tube.