The plans says to sand a curve along the upper longeron to match one of several templates. By holding up the templates to the curve you can get close. As you get closer to the final shape it becomes more difficult to determine where the high spots are that still need to be sanded to match the curve.

Rub a pencil along the template profile to lay down some lead. Then wiggle the template along the curve. The template will leave marks indicating where the high spots are that need to be sanded.

Here's a short 1 minute video demonstrating the technique.

Link to this page:
http://www.cozy1200.com/geeklog/article.php?story=SandingCurves
or
http://tinyurl.com/6oo3eg