I made these slipcovers without a pattern and measured them with the pillow. With a seam allowance of an 1" on each side so that they would have a looser fit.
Enough fabric to overlap with an inch seam allowance on each side
1-2 snaps (optional)
1. First fold over the top seam 1" and press and sew.
2. Fold over the opposite side, press and sew a 1" seam.
3. I make sure that the two seams overlap and then I mark where to cut the excess fabric and sew the side seams.
4. Turn the fabric inside out and trim at the marks, then sew 1" seam on both sides.
5. Turn so that fabric is right-side out and press. You can sew an optional snap to the inside flap so that the pillow cover stays closed and if like me you have little boys who like to take everything apart it will give you time to walk in on them trying to take the pillow out.