Any questions just ask. I used card stock wrapped around and lined up to get a straight line, then scribed with a pencil, then taped that line. I would use a hack saw if I had it to do over agian. Just nick it all the way around to get a good line and then start cutting in. Go at least a 1/8th over you desired length, because thats room to fix a boo-boo and if not a 1/8th wont hurt. Once you've cut it take a bastard file and file at a 45 degree angle to break the edge then come over with some emery/sand cloth to get that smooth look.
Do you know the dowel rod measuring method?
p.s. Cold blue works, but I'll be buying the paste next time I need some. The liquid is hard to applie precisely. However if you are only cutting you barrel you can just dip the barrel in it and hold for 30 seconds, pull it out for 30 seconds then wipe dry, rinse. Let sit for 5 minutes and then decide if you want another round. (not the instructions, but easier...) I had issues with getting it around the reciever where I filed the edges and it wouldn't stay. It took 3-4 times to darken it up.