Have a friend stand to the side of you and mark your arrow as close to your rest at full draw that is safe and provides clearance for your intentions, broadheads etc. Then do some quick measurements and weights using your current total arrow weight and subtract the amount you are going to cut off, (excess x gpi of shaft). Make sure you stay within IBO regs 5 grains / pound of draw weight. 60#'s 300 grain arrow etc.

A high speed cutter is preferred and something toensure a level cut. Glue to finish the arrow. Just about all you need, oh and be careful!