Try putting a small band clamp around the steerer tube to use as a cutting guide. Works great and serves the same function as tape. I clean the end up with a fine file when I'm done.
Originally Posted by I_bRAD
Fine tooth hacksaw and a steady hand. Deburred the edge with a dremel. (although from what I hear about CF dust I probably should have worn a mask. I'm sure I've done worse to my lungs though!)