I use a very easy, fool proof method to insert the zip. Tutorial below....
Firstly determine the size you want your cushions. Mine above were dependant upon the size of the remnant, but I could get 4x squares 37cm x 37cm for two cushions, but it could be any size you like... ;)
Neaten one side of each pair (I used the overlocker) and pin together. I like to mark in from both sides at 5cm, using this as the stopping point for the opening, but the choice is yours.
You will stitch the two sides together at your seam allowance, but change the stitch length between the dots.
Start from the edge in a small stitch and back stitch when you get to the dot. DON'T TRIM THE THREAD. Increase the stitch length to maximum, or use a tacking stitch (I have a button on my machine) and continue to the dot on the other side. Return your stitch length back to what it was before, back stitch and continue back to the end.
Press the seam open.
Lay the zip along the edge of the seam and stitch one side down.
The seam is still closed at this point. Fold it back right sides together and stitch all around. I like to trim back the teeth of the zip that are going to be stitched over, plus any metal stoppers that could interfere with stitching or overlocking. Stitch around the other 3x sides, overlock and trim the corners.
Next, get a seam ripper and clip some of the tacking stitches near the zip pull, at the opened side.
Open to the point where you can turn the whole cover right side out. You can then snip the tacking stitches back to where we we did those back stitches. DO NOT CLIP THROUGH THESE! If you do, the seam will unravel, and it could mean extra top stitching to stop it from coming apart.
And that is it! Simple, hey? The zip will lay on the seam, but then you can use both sides of the cushion. The zipper pull can tuck away into the end of the seam, if necessary.
Any size will work with this method, and it is fool-proof! I will definitely be making more. It's a good excuse to use up some more scraps and practice more techniques. ;)