My insert measurement for this pillow is 16x16. So, I cut out the front panel to measure 17x17, this gives me a half inch seam allowance all the way around. For the backside of the pillow, I cut out two panels, each measuring 17 inches high and 15 inches wide. Turn the edges over a half inch (press) and again another half inch (press.) Once you have pressed the panels, pin (if necessary) and stitch the edges. This gives each back panel a nice clean finished edge. Next, lay your fabrics right side to right side, overlapping your two back panels to align with the front panel of the pillow, pin and stitch all the way around. Finally, turn your fabric right side out and insert your pillow. See, wasn't that easy? If my directions made no sense at all please let me know.
Tip: When I was in college I thought pressing was so over-rated. Now I know that if seams are pressed really well, there is almost no need to pin things in place. It really does save time in the long run!