French fly, or waist stay, for trousers/shorts
Added Apr 1, 2012
Cut two long pieces of sturdy woven fabric on the grain lengthwise to be the hidden "belt" or waist stay (I'm just going to go with "belt" from now on, OK?) Each piece needs to be plenty long enough to go around half your waist, plus extra, and the width should be double your desired finished belt width plus double seam allowances.
Fold both pieces in half lengthwise right sides together, sew together one of the short ends, turn right side out and press. Press the whole length in half lengthwise.