Turn one leg right side out. Insert this leg into the wrong side out leg. Pin remaining crotch curve together. Sew back crotch curve, continuing stitching from front crotch curve. Clip curves and finish seam.
Boxers are now ready for the elastic casing at waist.
Insert elastic into casing. Try on shorts to determine how snug you would like them to fit. If the boxers are for sleeping, elastic should be loser. Teenagers like to wear them a little lower than the waist so longer elastic is preferable. If you are making the boxers for a man, use a pair he owns as your guide to how tight to pull in the elastic.