Make a stencil. Tape some butcher paper or engineering drawing paper over the car and make some marks on the paper in key locations for alignment, for example the marker lights and the trunk opening. Then trace the outline on the paper. Fold the paper up and store it somewhere safe and dry. To transfer the pattern back on you do a "pounce" pattern. Watch this video starting at 1 minute, you will get the idea.
[ame="https://www.youtube.com/watch?v=h-fDr6AqSEA"]Crusader Signs - Making a Pounce Pattern - YouTube[/ame]