The front piece of boxing is 54" long, the front of the cushion is 40" long so I had fourteen inches left over.  Divide that by 2 and seven inches were left over for each side.  The zipper is also 40" and goes across the whole back of the cushion.  If your cushion is shorter, you'll have more inches to wrap around.  Get it?  Good luck and you'll be so proud.

Thanks for the tips. I'm just going through all the steps and trying to translate the measurments for my own cushions. Is there a reason you wrapped the front around 7" on each side? I mean, how did you determine the number value? Or is it just a good starting point aand not really based on any ratio necessarily? Also --- the backpiece is the same length as the front?

I personally don't like cording either.  This was for my nephew's room and it needed some contrast.

What you can do is leave the piping out, just stitch the boxing to the cushion top (and then to the bottom) To make it look really professional, after you stitch the boxing to the cushion top and bottom, turn your cushion cover right side out and do a TOPSTITCH 1/4" away from the seam, on the TOP of the cushion, making sure the seam allowance underneath is folded towards the top, so that your topstitching will "catch" when you are sewing on the top.  Stitching that seam allowance in place by doing the topstitch gives it a nice, professional and modern look--I love it.  Be sure to keep your stitching really straight--You'll love it too!!!  Add a comment if you get stuck.

Well I know what I'm doing this weekend..... though I'm pretty scared!!

Is there some kind of alternative to the piping? I really find it tacky.... but I see that it helps keep the form. Hmm....