Place a nail at the beginning, the end, and the point of maximum displacement of the curve (defining the chord of the arc.)

Erect a parallel to the chord at the point of maximum displacement. Place a fourth nail on that line.

Fix together two straight pieces of rigid scrap so that one piece lies against the point of max. displacement (at one end) and on the end of the curve at the other. While the other lies on the parallel line; at the point of max. displ. 

Remove the fourth nail.
A pencil/scribe held at the crook between the two pieces of scrap. . . 

. . .will describe a uniform curve (circular arc segment) when the edges are guided against the nails.

Flip the sticks over to mark the other half of the arc.