First determine how long you want the sheet to be and mark the tube. Keep in mind the cutting area of your saw. A hack saw can cut a much longer piece of copper pipe than my short jewelers saw.
Start wrapping a piece of paper around the pipe at your mark. Line up the paper so it meets its beginning point. This should be a nice even line. Now, mark around the tube or tape the paper in place.
Cutting with a jewelry saw has taught me some very important universal lessens about saws. Jewelers saws have very delicate blades that will snap if twisted, or if you put too much pressure against the object you are cutting. First, relax- let the teeth do the work not your arm, use a sharp blade, use the full length of the blade whenever possible, pay more attention to where the cut is going rather than where it has been, and keep the cutting movement smooth and even.