1. Hold the tubing in your left hand and the measuring tape in your right. Hold the location with your thumb; then mark it using a permanent maker or pencil.
2. Tighten the wheel cutter to clamp it onto the tube at the mark, and rotate the cutter with the wheel following the rollers. work slowly for the first couple of revolutions to make sure the cutter doesn't slip out of the cut.
3. Use a thumb cutter when you need to cut copper space. These tools don't provide the same mechanical advantage as longer cutters, so they are harder to tighten and to turn.