Takes practice, and time.. (and a good sharp knife!)
With a good, sharp X-acto type knife, here's what I do:
(Assuming you want a shorter tube, trying to cut a tube to a desired length. The tube wont' be cut in one shot, but with repeated score marks, and you'll go around the tube a few times..)
Wrap the tube with a couple of wraps of thick paper at the place you want to cut. These wraps are your guide to the cut. The thick paper will give you an edge; a guide for the knife.
Carefully start the cut. Score, don't cut thru, with the tip of the knife against the paper wrap. Make about a 1/4 - 3/4" score, and then rotate the tube, and repeat. (you'll be doing this a lot!)
Continue scoring the tube. Depending on the skill, the thickness of the tube, if it's raining, if you have heavy metal playing, etc.. this scoring may take two or three (or more ) passes around the tube. (once you've gone around the tube about twice, you'll no longer really need the paper wrap / guide.)
You'll soon notice that you have actually cut thru the tube in a few places, keep scoring. The final pass around the tube, you can just cut thru to separate the two pieces.
Sand the cut edge to remove any chads / shards / burrs, etc..
Many people try to cut the tube in one shot, and they end up crushing the tube, separating the inner layers from the tube, and/or they try to make free-hand cuts on a marked line and not get a straight-90° cut. Take your time!