Align the elastic along the edge of the sheet, about 1/2 inch from the edge.

Secure the elastic with an anchor stitch by sewing back and forth a few times to hold it in place and prevent the elastic from shifting as you sew.

Start folding the fabric over the elastic to create a casing. Fold the fabric far enough to cover the elastic.

Sew along the folded edge with a straight stitch, gently stretching the elastic as you go.

Work in small sections as you fold, stretch, and sew.