Make three equal divisions between A and B; set apex C at 1/3 of AB and divide the vertical into three equal divisions
Set divider to the second division and step in from A and B to find foci D and E
Swing two circles at D and E with the divider set to A and B, respectively
Extend a line between the first vertical division and one of the foci; erect a perpendicular and mark focus F
Extend lines through the circles from F through foci D and E
Swing from F with the divider set to the far side of the circle to complete the elliptical arch