You are probably looking at this photo, thinking, my goodness, that is the ugliest pillow ever. And you would be right, if I were actually making this pillow! I chose two different prints and a lavender zipper so it would be easy to see which piece is which and you could tell right sides from wrong sides. In my examples, the small floral print is the 16" square pieces and the pink print is the 16" x 4" rectangle.
Place rectangle piece on top of square piece right sides together. (If your print is directional, the bottom of the square should line up with the top of the rectangle.) On the rectangle piece, mark a line 1 1/2" from the edge. Center your zipper on this line and make a vertical line at the zipper head and the zipper stop. Remove the zipper and pin the pieces together.