Swing arcs from A and B with divider set to A-B; mark intersection C
With the same divider setting swing from C; mark intersections E, F
Extend line from A→F, mark H; from B→E, mark G
Set divider to A-G; swing from focus G, mark J, swing from focus H, mark K
Connect points A→J, J→K, K→B. AJKB is a square