Cutting is the easy part. Having the line to cut is the hard part. In my
high school drafting class I learned how to layout a piece of paper with the
line on it. Cut out the paper and wrap around the pipe. trace line and
you're good to go.

I searched the web for instructions on this. its easy but a bit more than I
can explain. I cam across this layout software that will give you one free
sample:
http://www.ezpipe.com/samples.htm

Karl