Learned how to do this at a quilt retreat in Michigan last year. It's a wonderful way to make a personalized pillow case. I make pillowcases to match the quilts I make, but even if you can't sew - these are easy enough to whip up in under an hour.
Step 1. Press 1 1/2 inch strp in half lengthwise with right sides out (length depends on how big the pillowcase is to be - this strip should be double the length of the opening)
Step 2. Place raw edge of above strip even with edge of large fabridc, salvage to salvage with right sides together. Place right side of 9 inch strip to above over the narrow strip. Pin and stich a scant 1/4 inch seam.
Step 3. Roll large fabric towards the seam. Fold 9 inch strip to raw edge of previous seam with wrong side out. stitch a full 1/4 " seam being careful not to sew any of the rolled fabric into the seam. Pull the rolled portion of barbric out of one end of the now 4 1/2 inch border. Press. NOTE - the raw edge of the seam is not visible now.
Step 4. With wrong sides together, stitch a scant 1/4 inch seam along the side starting at the boarder end. Press seam open. Turn inside out and stitch a full 1/4 inch seam along the side and end of pillowcase. Turn right sides out and press. You now have completed a pillowcase with French seams !!!
For a king size pillow - a yard of fabric will do nicely for the body of the pillow. The 1 1/2 inch strip should be 44/45 inches long, same with the 9 inch strip. I usually cut these two strips before I start cutting into my material for piecing a quilt. Sorta like borders on a quilt - it's easier to cut them first so you don't have to piece them later.