Unwrap your pillow
from its cover, and fold it back up again, but this time, inside out.
Use your pin markings to find where it should fold.
In this case, you want your embroidered flap underneath, so it ends
up on top when we turn this whole thing right side out. If your pillow
is square, your whole folded fabric should be square. It seems obvious,
but it’s a good way to make sure it’s folded tight enough. Once it’s
ready, pin in place and sew a seam down both sides.