Well, there's quite a few ways- if you have access to a machine shop, no problem... if it's a metal tube- access to a tubing cutter- or a large lathe... sonotube, well, people are sometimes at a loss as to how to cut it.
If you're stuck doing it by hand, all is not lost- you can do a very good job with just a hacksaw. I've cut up to 6" aluminum tubing this way- and 10" Yazoo Paper tubing with 1/4" wall thickness. In all cases, the results were perfect. All it takes is some time- and patience.
I'm currently putting together a 100mm f/13 Carton-lensed refractor, and thought I'd grab my camera and take a few pictures as I worked. Hopefully, this will encourage someone that they can do it themselves.
You need to accurately mark the tube first- this is easily accomplished by wrapping the tube in a piece of paper- you may have to tape 2 sheets end-to-end- and matching up the edges- then taping the paper to itself so you now have a paper sleeve on the tube- and the ends are your marking guide. The paper sleeve should be snug on the tube, and you should have to really work to slide the sleeve along the tubing.
You can then take a Sharpie and run a line around the tube using the edge of the paper to guide the pen.
Then you can slowly and carefully start your cut with a standard framed hacksaw with a fresh blade. Work carefully to start the cut, then- as you get the cut started, slowly roll the tubing a little in one direction or the other- to move the cut along your line. You want to keep the blade in the groove you've started, yet move the blade along your line, extending your groove just a little at a time. In this case, I worked on a tall garbage can with a doubled over work quilt thrown over the top- and then a piece of cardboard. I just keep the saw carefully going, rocking the tube back and forth. When I have a nice groove marked and established over 20-30 degrees of the tube, I'll stop- roll the tube a little- and continue working the groove a little further and further as I rock the tube.
Finally, I've got a cut line established all the way around the tube.
Please note- the cutline is to the right of the hacksaw blade in this picture.