Here's a simple illustration on how to make and use a small plywood form for supporting a dry stone arch until it supports itself. Most arches have something to hold them up until they are made. This one is a Gothic arch form. You could make a Roman arch or an elliptical arch the same way.
By the way, don't confuse elliptical arches with electrical arcs. Even though they sometimes have similar shapes, one is kind of 'magical' (the way the stones hold together) but the other, if your holding live wires close together, is just 'shocking'.