There are three ways to trace a shape, contour or other non-straight line prior to cutting materials to fit. One of them is to guess at the shape, cut material and then keep making cuts until it fits. This is the wrong way, and we've all done it.
That leaves two, much better, ways to trace a shape. The easiest is to buy and use a contour gauge. These devices come in metal and plastic and in lengths up to about 18 inches. We don't like the metal ones because their "bristles" tend to bend, they eventually rust and the tips of the metal bristles scratch the materials they are pressed against. Plastic gauges are the best.
Using a contour gauge is easy: just press the fingers against the shape while holding the gauge 90-degrees to the surface. Once all the fingers are pushed down tight you can lift the gauge and trace the shape. Piece of cake!
Using a contour gauge is pretty simple...just move the fingers until they conform to the shape.
The third method is for those who don't have a contour gauge. It is the venerable, seldom-used, lost-in-the-drawer compass. Everyone has one, usually the one left over from high school, but only use it when a circle needs to be drawn. The compass makes a great shape-copier.
Before you use your compass to trace the contour, wrap the point in tape to prevent it from scratching.
Don't have a compass? Well, take two pencils and hold the eraser ends touching. Spread the tips apart and put a clamp on the eraser ends. Voila, you have a compass!
Using two pencils to trace the contour. The important thing here, of course, is to maintain the distance between the two pencil points.