What you need:
some plain old denim shorts with a straight hem
a sewing machine
What you do:
1. Cut out a scallop shape from some cardboard, like the one in the picture below.
2. Fold the edge on your shorts up approximately the height of your cardboard stencil, on the RIGHT side of the shorts.
3. Trace around the scallop shape, making scallops all the way around the cuff of your shorts. Pin the folded up bit to your shorts so it doesn't move when you start sewing.
4. Using a small straight stitch, sew around your drawn lines. Lift and readjust the foot as you go to make it easier!
5. Cut out your scallops, just outside the line you sewed. Now iron, flip the hem inside out, iron again, and you're done!