Measure your pillow or the old pillow case and add 1" to each side. My old pillow case was 26'x 27' so I cut two squares of fabric that were 27" x 28".
Lay the fabric face down and fold the side you want to be the bottom in about an inch (just big enough to accommodate the width of your zipper). Do the same to both squares of fabric (this is where your 1" seam allowance comes into play). Iron down the folded sides.
Place the zipper centered on your fold with the zipper pull face down like the orientation of the fabric, making sure to get the edge of the fabric as close to the zipper as possible and pin in place (image 1). When you flip it over it should look like image 2. Using the "zipper foot" attachment of your sewing machine, sew one side to the zipper, removing the pins as you go. Stop when you get to the end of the zipper. Do the same with the other side. When you have your two panels sewn to the zipper, finish the ends by pressing the folded sides together and sew closed.
Place your squares with their "right" sides together and sew the remaining sides, leaving a small margin. BEFORE you do so, UNZIP the zipper halfway, otherwise you won't be able to turn your pillow right side out when you're finished. Iron after you turn your finished pillow right side out.
Stuff your beautiful pillow case :)