Draw a vertical line AB.
With the centre at A, draw an arc.
With centre at C, draw an arc intersecting the arc at D and E.
Join AD and AE and extend these lines.

Take points F on AD and G on AE such that AF = AG.
Let M and N be the midpoints of AF and AG, respectively.
Using a compass, draw semicircles on the lines MF, AM, AN, and NG.
Erase the extra letters, lines, and arcs to get the required arch design.