First you need to measure your cushion to see what size you need, alternatively if you already have the old cushion cover you can measure that.
Now depending on how many panels you have you need to work out how wide they need to be then add a half inch each side plus an inch to the length for your seam allowance ( it will be a half inch seam allowance throughout).
You could also just play around with your fabric and lay it on, make sure your cushion is covered with enough over hang for seams, sew them all together then make your neat fitting square once you have done this. This may be preferable for anyone who is put off by adding it all up and working it out, after all the cushion is unique it doesn't have to follow any set rules.
That is what I did in the one above.
You then need to press your seams open, don't skip this bit as it will not look right otherwise.
Then you are going to add a zip, I recycled the ones from the old cushion covers as its always best to use what you have , just carefully unpick, being super careful at the ends as in my case the zipper had already been trimmed and could fly off the end if you are not careful. Pull out all the loose threads. Lay it on your two cushion sides which should be facing right sides together, wrong side up facing you.
I mark at each end with a pin as in the photo. Now remove the zip for now, and sew a half inch seam down to the pin stop and reinforce, go down to the other end where the other pin is and sew a half inch seam again up to the edge.
Now you will have a gap with no stitching down the middle, turn your stitch on your machine to the longest length and sew all along between the pins keep it loose don't reinforce as you will be unpicking shortly.
Now open that seam flat, add your zipper foot to your machine, lay the zip down right side facing downwards and sew closely to teeth, you will know where to position it as you have your pins in place still.
Repeat up the other side.
Now you have in your zip but no opening! take your seam ripper or small scissors and release that loose seam up to the reinforced stitching at each end of the zip. This should have ensured your zip is in nice and even on both sides. You can top stitch at this point if you like on the right side, I didn't in this case.
With right sides together sew all around the cushion using half an inch seam allowance ( did I mention make sure your zip is in the open position first) You now have a nice cushion shape, snip off the corners then zig zag or overlock all around to prevent fraying.
Now neaten all edges, get rid of al those stray threads and turn through. put it on your cushion and admire your work. I know there will be people out there saying I don't do it that way, well that's fine, we all find our own way. Also not all my cushions are made this way, but some are, it's quick. I also use envelope backs, concealed zips, plackets and velcro but that is something for another time. There may be a lot of photos but this really is a quick way to do it, have a go and let me know what you think. Here is the cushion below.
The leaf fabric on the cushion is left over from a skirt I made so when I wear it I will be blending in with the furniture! Back soon with some holiday images and great finds from the charity shops there.