So, here goes!
Firstly, let me start by saying that my front and back squares started at 52cm square, with the border strips at 7cm wide and about 70cm long (I didn't measure it exactly, as long as it's longer than the width of the front plus the border). The exact measurements aren't really relevant, unless you want to make an exact replica. ;) I allowed for a 1cm seam allowance on all pieces. Oh, and you'll need a zip. Longer is better, since you can trim it back if necessary.
I have here the front and back, plus 8 strips for the border edge (4x for the front, 4x for the back). You can see that these are wider than the base pieces. I didn't measure them exactly, since I was using scraps, but they are 7cm wide.
The zip will be inserted first in the seam at the bottom of the back piece. Firstly, I folded the front pieces in half and notched all the halfway points, and did the same on both sides of the border pieces. It's easier to do this first, and keeps it accurate.
I also marked the corner of the "Y" seams (which I should have done before the zip went in, but I forgot and had to unpick a bit later on). It is easier to set in the zip if it does not go to the seam edge, so I marked in from the edge, first 5cm, then I increased it to 6cm (which is why there are 2 marks). It was a bit of trial and error, and all measurements are what I thought would be best, but are not set in stone.... Stitch from the corner dot to the edge of the mark, on both sides.
Iron back the seam allowance, making it nice and crisp.
This will make it easier to insert the zip. Since I will be top stitching around the seam once it is done, I needed the zip to be off centre. This means that the seams will not be the same on both sides of the zip. This is a little hard to explain.....
First, clip the seam at the mark, only on the side that will fold up. The other pieces will fold towards the border.
I pinned the zip in the position that I wanted, and used that as a guide as to where to stitch down. I want to stitch the covered side first, then the side that butted against the zip.
Using a zipper foot, I stitched close to the zipper teeth.
Then I did the same with the other side. This one worked out that the edge would match the edge of the webbing. Again I stitched close to the teeth with my machine zipper foot.
At this point, turn it over and double check that it sits nice and flat. I then stitched across the extra length of the zip a couple of times to secure it, and trimmed it off.
It was then time to top stitch the covered side. I started at the seam, over the zip and down the edge of the covered side and back to the centre. I used the open toe foot on my machine, but I could have used the zip foot if I wanted it to finish wider.... Stitch a section with the zip puller ahead of the foot, and then manoeuvre it under the foot with the needle down, and continue on.
Now that the zip is in, we can go ahead with the "Y" seams, or mitred corners.... ;)
Match up the notches made earlier with one side of a border strip and pin. Flatten out the piece and pin at either end. Stitch with the back piece facing up, so you can see the dots you made earlier. Continue with all sides of front and back pieces, starting and stopping at the dots. Make sure the underneath piece is out if the way when you back stitch.
Stitch all corners on the front and back pieces. Turn over and press flat. If it doesn't sit flat, it means that the angle was not accurate, and you can simply restitch at a steeper or shallower angle, as required. Make sure you unpick the unwanted row of stitching, or you won't get a flat seam.
Once you are happy with all the corners, trim back the seams, and press really well till they are nice and crisp.
Trim the corners before turning right side out.
Now, you could stop here, if you want to. It would just be a big pillow with a small zip opening, but my plan was to stitch down the flanged edge, so once it was turned and I was happy with the corners, I pressed the seams flat and used LOTS of pins to keep it together at the seams. I worked from the back and planned to stitch in the ditch, but this isn't so easy when you can't see where you are stitching. Using the pins as a guide, I stitched as best I could in the ditch, or slightly on the main body, rather than the border strip.
The final result is by no means perfect, but it looks pretty good to me ;)