Measure the length of the tube you need. Take the metal tubing and mark the length that you need to cut.
Take the tubing cutter and expand the jaws by turning the screw on it to accept the diameter of the tubing you are going to cut. Place the cutting wheel on the mark and tighten the jaws again to hold the tubing in the jaws firmly.
Rotate the cutter around the tubing to scribe a line at the point where the cut needs to be. Tighten the screw about 1/4 to 1/2 a turn, then rotate around the tubing again. Each time you do this, it makes the cut a bit deeper.
Continue this process until the wheel cuts all the way through the tubing wall. If you notice, the end of the tubing is slightly narrowed in this process.
Twist a drill bit around the opening of the tubing piece to clear the edge from the tube. This is done also to remove any burrs that may be left behind from the cutting process.