One of the most common ways to mark large circles and arcs onto sheets of lattice or plywood is with a pencil tied to a string. Unfortunately, it’s also one of the most inaccurate methods because the string easily stretches.
To more precisely scribe circles, take a metal shelving standard and lay it on the workpiece with its slotted face pointing up. Tap a nail through a slot at one end of the standard to act as the pivot point. Measure up from the nail a distance equal to the radius of the circle. Insert a pencil through one of the slots at that dimension. To scribe the circle, simply swing the standard around the nail.