The hem itself is actually a large panel constructed of a 1.5" width of grosgrain ribbon sewn to a 3" width of cotton fabric. The length will be equal to the circumference of the bottom of the pants leg plus at least a 1/4 of an inch for a seam allowance. You will be sewing through heavy layers so use a heavy duty or denim needle on your machine.
1) Decide where you want the bottom hem of the pants to hit on your leg, remember you will be attaching the fabric hem to the pants fabric itself so you can ultimately turn them up like I do in the first photo. For example, when I wear these uncuffed, the bottom hem of the fabric and jeans combo hits me at the ankle. So if you like this look, trim only enough of the pants off at the bottom to reach to your ankle. The important thing to remove in this process is the thick original hem of the pants.
2) Pin the ribbon to the fabric with a 1/4" allowance of the fabric behind the ribbon. (Fraying won't show) Run a seam straight across the bottom edge of the ribbon to attach these together. Place it around the pants leg to determine where the sides will meet, pin and remove from pants. Stitch this seam closed to form a tube.
3) Turn the pant leg inside out. Position your fabric panel tube so the bottom edge of the fabric (not the ribbon) lines up with the bottom of the pants. The fabric will be wrong side up. Pin in place and sew together with a 1/2" allowance. Turn the fabric tube outward and press the seam flat so you have a crisp edge of about 1/2" of fabric showing on the inside. For those of you who have sewn quilt binding, this should be familiar.
4) Now turn the pants leg right side out. Flip the fabric tube over the bottom of the pants and pin in place. Sew together with another straight seam across the top of the ribbon edge this time. The ribbon gives a stiffness and weight to the upturned cuff that you see in the first photo. Press to sharpen the edges and set the seams.
5) Enjoy your new sassy pants.