Fold over fabric top 1/4” and press. Omit this step with knits—they don’t ravel!
Fold over again 1-3/4” (or as specified in your project) and press.
Edgestitch as close as possible to edge of bottom fold, leaving a 1-5/8” area unstitched (or as specified in your project).
Cut 1-1/2"-wide elastic (or width specified in your project) to desired length. Secure large safety pin to one end of elastic. Thread safety pin through one side of casing opening.
Keeping elastic flat (no twists allowed), push safety pin leader through casing until pinned end of elastic comes out at opposite side of opening.
Pull both ends of elastic strip out and away from casing.
Fold a fabric scrap in half. (1-1/2”-wide elastic requires a 1-1/2x3” scrap; 2”-wide elastic, a 2x4” scrap.) Butt ends of elastic and pin to square.
Sew elastic to square, first in a rectangle, and then in an X.
Work elastic back into casing.
Stitch opening closed.