A thin scrap flexed into a curve is a great way to create templates for drawing arcs. The challenge is how to hold the curvature just where you want it while you trace the shape. I’ve seen a few approaches to this jig over the years, but I recently came up with my own simple solution.
Just pull the drawstring out of the bottom of an old coat. Use the kind that has a push-button plastic cinch attached to it. Thread this string through a hole in each end of your bow and knot one end, then slip the cinch onto the other end.
Now, you can draw up the string to change the arc and lock the shape with the cinch wherever you like.
– Jacques Joanette