Encasing the Elastic
All your sheet needs now is a hem and something to hold the elastic in place around the head and the foot of the bed. The easiest way to do this is to hem the sheet and use the hem as a casing. Fold the entire edge of the sheet under ¾″. Open this out, and press the raw edge under so it meets this first crease line. Stitch the hem all the way around the sheet close to the inside fold.
Cut the elastic into two 30″ strips or the width of your mattress plus” 2 or 3″. You will be threading one through the hem at each end of the sheet. Begin by locating the points on the long sides of the sheet 3″ from the corner seams. Mark these points on the hem allowance.
With a seam ripper, take out about three stitches in the hem at these points. Fasten a safety pin to one end of a piece of elastic. Thread it through the hole in the hem and toward the corner seam. Be sure to secure the tail of the elastic to the sheet with a pin before you lose it inside the hem/casing.
Pull the pin and the end of the elastic out of the hole in the hem beyond the second seam. With the fabric adjusted so the elastic won't pull away, line up the end with the 3″ mark. Machine-stitch and backstitch across the hem and elastic ¼″ from the end of the elastic and again about ½″ from the end. Do the same to the other end of elastic, and then repeat with the other elastic strip at the other end of the sheet.
Repair the hem by stitching over the gap and about ½″ or so on both sides of it, and your sheet is done.