I've got a really straightforward method for putting a zip into the backing fabric for your cushion so thought I would share it with you. As with all things though, what's simple and fairly quick when I'm doing it, becomes a lot more confusing when I'm trying to explain it. I've included lots of pictures which will hopefully help to make it clear, (although as I was making a white cushion back with white thread and a white zip it wasn't ideal) but if you have any queries then just ask.
What you need:
Your cushion front.
A piece of fabric for your cushion back, a bit bigger than your cushion front.
A long zip (I use about a 14" zip for a 16" square cushion).
1. Measure your cushion front. Add one inch to the measurement for one side, and three inches to the measurement for the other, and cut your backing fabric to that size, eg if you have a 16" x 16" cushion, cut your backing fabric to 17" x 19"; for an 18" x 18" cushion, cut 19" x 21" etc.
2. Turn your fabric so that the "long" edge (ie the edge you added three inches to the measurement for) is vertical. Fold approximately the top third of the fabric down, and iron in a crease.
4. Sew about 2-3 inches (depending on your cushion and zip sizes) inwards from each edge along the crease you made in step 3. The gap between the two lines of stitching should be a bit shorter than your zip.
6. Iron the seams you have just sewn (and cut) open, but iron along the whole width of the fabric (including the part in the middle that wasn't sewn). If you lay your fabric down the right way up, it should have a slit running across, about a third of the way down from the top.
Once I've gone down one side I turn, sew across the end of the zip, then turn and sew up the other side. When going up the second side of the zip, I pull the fabric over the zip so that it overlaps with the fabric on the other side very slightly (as in the photo below) - this makes sure that the fabric on the two sides of the zip butts up against each other and makes the zip almost invisible.
I hope you enjoy the tutorial, let me know if you have any issues!