Cut from thin plywood or sheet metal. The template defines the intrados (bottom curve) of the arch. As the mason lays each voussoir, the template slides along to check the curve.
Why learn arch craft today?
The Romans realized that a true semicircular arch made of wedge stones could span vast distances. The became the backbone of the Aqua Claudia and the Colosseum . Roman engineers invented the centering —a temporary wooden frame that held the voussoirs in place until the keystone was driven home.
You will fail the first time. The keystone will be too loose or too tight. That is fine. The arch craft is learned through failure.