I make a truck load of cushions with zips throughout the year and experience has taught me that making them with a double layer really helps to keep them nice and firm and maintain their shape. If you have found it difficult to make backs with zips, well let me make it easy for you . . . it really is much easier and quicker then making backs with buttons!
You will need some nice backing fabric and some cheaper homespun fabric, this won't get seen so you can use anything!
For a 18" cushion cut 1 piece of each @ 19" x 16 1/2" + 1 piece of each @ 19" x 5"
For a 20" cushion cut 1 piece of each @ 21" x 18 1/2" + 1 piece of each @ 21" x 5"
Take the larger backing and homespun fabrics and place them right sides together. Pin and machine sew along one long edge. Repeat wit the two smaller backing pieces.
STEP TWO + THREE
Take the larger backing pieces, fold out the fabric so that the right sides are facing outward and press along the seam. Repeat with the smaller backing pieces. Top stitch 1/8" from the seam to only the larger backing pieces. (3)
Take the smaller backing pieces and place the zip, right side facing down onto the right side of the fabric lining up the top edge of the zipper tape with the seam. Pin in place. Using a zipper foot on your machine sew along the top edge of the zipper (about 1/4" from the edge of the tape)
STEP FIVE + SIX
Fold the zipper up and top stitch to hold down the seam - make sure the zip is not fold upward towards the zip!
To the back of the larger backing piece draw a line across the panel 2" up from the seam.
Line up the top edge of the zip (zip face down) along the drawn line as shown. Pin in place. Using the zipper foot on your machine, sew along the edge of the zip guiding the edge of the machine foot along the edge of the zipper tape. It is important to keep an equal distance from the drawn line / edge of zipper tape to ensure the stitching looks nice and straight from the front. Also note that the bobbin thread is what you see from the front so make sure you have loaded the correct colour onto the bobbin.
TIP: if you are having trouble getting past the zipper pull, make sure your needle is in the down position, lift the machine foot and move the zipper down the zip past the machine foot. Put the presser foot back down and continue sewing.
You should now have a complete back with a nice folded flap covering the zip ready to be sewn onto your finished cushion top.
Follow your pattern instructions on how to add the back to the cushion front - this will depend on whether you are adding a binding or not. . . and make sure when you do sew the back on that you make sure the flap is positioned at the bottom of the cushion!
A NOTE ABOUT ZIPS
If I am adding a binding to a cushion, I quite often use a zip bigger then the width of the cushion back. It rids you of the worry of accurately lining up the zip ends and of sewing over metal bits when sewing on the back . . . you do however, need to make sure you have pulled the zipper into the middle of the cushion when you sew the back to the front. Its fine to sew over the zipper teeth (as long as they are not metal teeth obviously!)
I hope that has made it all nice and simple for you! good luck and happy stitching!