I want to make a pattern of a car bumper.  I need to calculate for
compound curves. How do I do this?  Once the pattern is made I will
take a digital picture of the pattern, and import it into a design
software program to be cut out by a plotter/cutter. I just don't know
what to use to make the
pattern or where to buy it. The pattern needs to be accurate to within
1/8".  The material I will be cutting the pattern out of is 3M Paint
Protection Film and it does stretch about 15%.  I have tried masking
tape and plastic film, but the compound curves just don't come out
right.

I think what I would try is fairly sturdy paper sprayed on one side
with re-positionable adhesive.  You can then put the paper over the
bumper, using folds and creases to get it to cover the surface
smoothly.  Cut around the edge and cut away the extra layers where the
paper is creased to follow a curve.  When you take the paper off the
bumper it should lay out flat again, with all the slits and notches
needed to exactly cover the bumper.

To pattern the bumper is not an easy task and the process can take
about 1 day even with the correct software. Your bumper if on a
standard road car may have already been templated as most are. To see
if your bumper has been templated previously you can look at
http://www.car-shine.co.uk and if it is there can be purchased by
contacting my company. We actually use VentureShield Film which is
easier to apply than 3M in our opinion. If the template is not there
you may like to contact VentureTape directly and can be done through
their web site at http://www.invisiblepatterns.com as they do provide
the software to make your own templates.
You can contact Car Shine UK directly if you wish on 0845 052 1241
Hope this is helpful!