Wrap a paper that is long enough to wrap 1 1/2 - 2 times around the pipe, this gives you a square guide to mark it. Then I use a battery powered circular saw to cut it, rolling the pipe while I cut it. Make sure the pipe is well supported at all times. Fast, easy way to do it. If metal pipe then mark it the same way and use a portaband to cut it.
Good luck
Ron