Measure your pillow width and add 1.5", this will be your fabric width. I had a 20" pillow insert so my measurement was 21.5"
Measure the height of you pillow and multiple that by 2.5, plus 4" for seams. This will be the length of your fabric. The height of my pillow was 20" so my length was 50+4=54".
On one short end of the fabric with the wrong side facing you, fold up 1.5" and then fold 1.5" again for a nice fat hem. Pin.
With the folded edge facing you. Sew along the hem using your presser foot as a guide to keep things straight.
On the opposite short end fold up one half inch and fold up one half inch again for a little hem. Pin and sew along the hem as above.
With right side facing you fold up the edge with the wider hem so the length of the fold is 3/4 the height of your pillow. So I folded mine up 15". Pin, right sides together.
Fold up the edge with the smaller hem so that the entire pillowcase now measures the height of your pillow. The wrong sides will all be facing you now. Pin. My measurements at this point were 20" top to bottom and 21.5" left to right. Little hem is on top and fat hem is hidden.
Sew each side with a straight seam and a 3/4" seam allowance. Then go over the extra fabric at the edges of the seam with a zigzag stitch for extra durability. Trim off any extra fabric being careful not to snip your zigzags.
(the extra fabric at the edge before I went over it with a zigzag)
Fold right side out, press, and stuff with your pillow insert.
Enjoy your new pillow!