Step 1) cut out your fabric like this
2 large square panels that are bigger than your pillow form by at least 1" on every side. Luckily my pillow had an old cover I could use as a pattern here. 1 piece of a complementary pattern which is as wide as the 2 large panels, but only 1/2 to 2/3 as long. And two pieces about 2 1/2" wide and 12" long.
Step 7) Lay the two square panels against each other (right-side-in) and match-up the edge you sewed in step 5 with the crease between the other panel and the complementary piece.
Pin the two large panels together and sew around the 3 edges that do not connect with the complementary piece.
Step 8) Turn the case right-side-out and put the pillow form into it.