I like my pillowcase a little bit smaller than my pillow form, so, I didn't add a seam allowance so it will looks full with no empty space.
From the top to the bottom is 23".
From the left to the right is 52" .
(I double the size of my pillow, 23 x 2 = 46, plus 6-inches for the hem for envelope opening and for overlap, so, I end up 52" in length).
Fold the edges of your fabric 1/2- inch and iron flat, then fold it over again 1/2-inch and iron it to create a clean hem. Hem the edges of your fabric. This will become your envelope opening.
Fold it into the shape for your pillowcase (right face together), allowing a 4-inches overlap of fabric on the back side of your pillow. Pin the open edges and stitch straight using 1/2-inch seam allowance, and zig zag to avoid fraying.
(Note: If your pillow is smaller than my pillow you can just use 2 to 3 inches overlap of fabric on the back side of your pillow.)
Turn the pillowcase right side out, then stuff with your pillow. And you are done!