How to make perfect corners
Using an iron, fold first fold of hem on each side to wrong side and press. Fold over hems again then press.
Unfold, and trim off corner of fabric at junction of first crease (see diagram, right).
Fold in trimmed corner towards centre to innermost crease.
Fold in sides to second crease; fold again (see diagrams, right).
Press the edges, pin in position and topstitch all around.