Fold the back pieces of the pillow over and pin into place. It should form a square the size of your pillow form (for me 18").
Sew along each side to seal the pillow case. Serge to finish if desired.
Turn right side out and stuff with pillow form.
Notes
Watch the full instructions in the video above. For a basic pillow case tutorial and how to change the dimensions to fit other sized pillow forms see my envelope pillowcase tutorial.