Tape the elastic to the end of a pencil or another straight object to guide the elastic.

Carefully insert the pencil and elastic into the casing through the opening you made.

Pin the back edge of the elastic to the sleep pants. This stops the entire elastic piece from going through the waistband.

Push the entire length of the pencil into the casing until the elastic starts going through the casing.

Continue to pull gently, until the pencil and elastic are out of the other side of the opening where you started.

Remove the pencil and tape from the elastic.