Note: I'm sure there are alot easy, more accurate ways to do this, most involving a table saw - which I don't have.
Step 1: Tape and Measure
Here you need to figure out where you want to cut. I needed my toms to retain their mounting hardware on one half, so I cut mine with a 1/4" clearance from the resonant side lugs.
Tape with Masking tape where your going to cut, it doesn't need to be perfectly straight.
Mark little dashes on the masking tape at the distance from the edge that the cut will be. Intervals should be at about 1".
Step 2: Connect the Dots
Step 3: Chop
This initial cut should only be deep enough to cut through the shell so there is a slit large enoug to insert the jigsaw blade.
Step 4: Chop 2.0
On my jigsaw, it was hard to see where the blade was cutting, so i flipped the blade backwards and cut backwards (most jigsaws aren't like this).
Step 5: Fin
A new wrap may be in order if you chipped it, or you can just cover the edge with a rubber hose.