Measure the width and height of the pillow insert.
Add the top and bottom seam allowances to the pillow height to calculate the fabric width.
Calculate the fabric length using three sections: 75% of the pillow width, 100% of the pillow width, and another 75%.
You can also multiply the pillow width by 2.5 to calculate the total fabric length.
Cut the fabric and mark the three folding sections.
Fold and press both short ends twice to hide the raw edges.
Sew a straight line along both finished hems.
Lay the fabric right side up and fold both end sections toward the center, overlapping them across the back.
Check that the folded width matches the width of the pillow insert.
Pin the top and bottom raw edges.
Sew both edges using a ½-inch or 1-centimeter seam allowance. Reinforce the areas near the envelope opening with a backstitch.
Finish the raw edges with a zigzag stitch or serger.
Turn the cover right side out and press it flat.
Insert the pillow and adjust the corners.