On my most recent project I needed an elliptical arc. Although my arc conforms to the Golden Ratio (1:1.62), the method I use works for an ellipse of any proportion.
Determine the overall size of the ellipse you need and cut a piece of pattern material large enough to encompass it. Next, draw vertical and horizontal lines that bisect the pattern stock. This divides the material into quarters; the arc is drawn in one quarter at a time.
Now make a simple scribing tool that allows you to draw the arc. Start with a thin strip of wood that is a little longer than half the longest dimension of the ellipse. Drill a hole near one end for a pencil point. From the pencil point, at a distance equal to half the height of the ellipse, drive a finish nail. Then drive another at a distance of half the width of the ellipse. Make sure to drive the nails so only about 1⁄16″ protrudes below your wooden strip.
(excerpted from the winning trick, by Bill Wells)
Read the entire winning trick here.