All right happy pillow maker. Here are your final instructions for assemblage.
For the sake of clarity I will label your three pillow pieces as A for the front 17" x 17" piece. B for the back piece that measures: 17" x 14" and C for the back pieces that measures 17" x 12".
Step 1: You want your inside of the fabric to be laying towards the table. Iron a 1.4" hem on B & C. Turn that hem under and iron another hem 1.4" wide. Your final hem should look like this:Pin your hem if you need to and sew a straight seam down the entire length.
Step 2: Place piece A wrong side down on the table. Place piece B on top of piece A with wrong side facing out and the hemmed edge towards the middle. Place piece C on the opposite side to form your envelope closure. And pin this together.
Step 3: sew a 1.4" seam around the entire length of the fabric. Keep you lines as straight as you can, especially at your corners.
Step 4: Remove your pins and turn your pillow case right side out. Place your pillow form inside and you are done.