Most towels are made of terrycloth or other highly absorbent materials. The hems eventually wear down and the threads break, causing otherwise usable towels to badly fray. Hemming the towels properly, whether you are making a new towel or fixing an existing one, gives them a clean edge with no lose threads. Trim off any fraying that has already occurred before you begin to hem, as the loose threads in the fray get tangled in the sewing machine.
Turn the edge of the towel in 1/4 inch. Turn in a second time, by 1/4 inch, trapping the 1/4 inch of raw edge on the inside of the fold.
Pin the fold in place. Place the pins perpendicular to the hem so your machine can stitch over the pins.
Stitch along the fold with a zig-zag stitch. Catch the top of the hem fold in the bottom point of the stitch and stitch through the fabric above the fold with top point of the stitch. A zig-zag stitch holds the hem flush to the towel. Begin and end the stitching with a backstitch so the thread doesn't pull loose.
Clip the threads once hemming is complete. Fold and hem the remaining sides of the towel, if necessary.