When cutting a piece of steel with a hand tool, you’ll need to put an accurate mark on it along the full length of the intended cut location. That is usually no problem with flat or square pieces. On those, it’s easy to mark a 90° cut line. Just place a square on it.
But that won’t work very well on round tubing. So, how can you make an accurate 90° cut line around the entire circumference of a piece of round tubing? Here is an easy and cheap way to do it. Find a piece of paper long enough to go around it at least one and one half times, wrap the paper around it so the overlapping edges of the paper are perfectly aligned, then draw the line.
In this picture a sheet of ordinary sandpaper was used instead of paper to do the job. †