I've never really got on well with the more usual envelope fold; I could never judge it just right so that it would sit right!  I started using a lapped zip instead; not that much more work, and sits perfectly!

Fabric requirements:
I usually make my cushions about 16 inches, give or take depending on the pattern.  I do this because it means that an 18" filler will be nice and plump.  It has an added advantage in that a fat quarter will make a back with this method!

Step one:

Cut the fat quarter in half, across the width.  (If not using a directional print, you could cut it from top to bottom which will yield a bigger back.  Turn the cut edge on each half under twice to encase the raw edge.  Tip: if you turn it just a little more than 1/4", you can use your usual 14" to hem!

Step two:

Use your zipper foot and position the zip underneath the hemmed edge.  Topstitch, sewing the zip underneath the hemmed edge.  Repeat on both sides of the zip.

Step three:

Fold the top half of the back down over the zip.  Press.  Fold it back up, covering the zip (I usually aim to cover the line of stitching from hemming earlier).  Press again.

Flip over to the back.  Your next step is to sew the folds you have just pressed in place.  Sew down the top side of the zip, securing the fold.  This line of stitching will be visible on the finished cushion, so try to keep it straight!   Then turn back over.  Topstitch the fold - this will leave it neat!

Et voila!