Locking pants are used for several different purposes. In certain circles they serve as fetish devices in domination games. Some locking pants are used in mental homes and nursing homes "on persons who remove their diapers...they help the caregiver to keep the patient's diapers on" according to Mountain View Adult Diaper Company. The pants are most often underwear made of plastic or rubber to prevent staining and to make the cleaning process easier, but you can create your own version using any material you like.
Find a pair of pants or underwear you'd like to work with. Underpants are typically used to create locking pants.
Cut a small, triangular opening in the back of the elastic. This should be near the very back where the tag is. This is where you'll insert the chain.
Fold the top portion of the underpants (the elastic in most cases) over itself. Bring the front down around the entire pair so that it loops back over.
Pin down the elastic to itself or to the very top of the material. Use enough pins on each side so that it stays in place.
Sew the top end of the elastic to the bottom end (or the top of the material). Go around the entire waist and secure everything but the opening you cut in the back.
Feed the chain through the elastic loop you've just created. Pull one end of the chain all the way through to the other side.
Attach the pad lock to the ends of the chain. Fasten the lock once someone is wearing the underpants.