They are literally driving me crazy because their style is pretty hard to upholster! But in the meantime, I was asked to upholster another chair seat that is much easier to do and it took me about an hour to make it.
This kind of seat is very common so I hope this Instructable will be useful to you!
Before I started to work on this seat, I removed everything that was around it because it was broken, so all that was left was a wood frame. In case yours is fine and all you have to do is covering it, go to Step 4.
Step 1: What You Need
Fold the beginning of it so that it can't break and, using the staple gun, attach it to about one half of the wood frame, as you can see in the pictures.
The webbing must arrive to the other side of the frame so cut it where the wood ends.
Pull the webbing as tight as you can and secure it with a few staples leaving a couple of cm (0.78 inches) free.
Like you did at the beginning, fold the end and attach it. The only difference is that it will be upside down but don't worry.
Do the same on the other half of the frame.
Now rotate the frame and repeat the same process so that there will be 2 new strips perpendicular to the first ones.
You can twist them like I did (see the pictures) but it's not really important.
It doesn't matter if the fabric you are using now is bad because it will be hidden.
Place it on the frame and, always folding the edges, attach it to the wood with the staple gun.
Take some upholstery batting and put it on the fabric, especially in the center, where there is no wood. This way you can give about 1cm (0.39 inches) of thickness to the middle of the seat.
Place it on the batting you added in the last step and start to attach the sides of it to the sides of the wood frame: first of all put 1 staple on each cardinal point, always pulling the sides a little. Then continue to add staples on the whole perimeter of the square.
When you arrive to the angles, cut them out and finish to attach the foam to the wood.
You must cut a piece of it that is big enough to cover the whole foam and arrive to the bottom of the seat. I'd say about 10cm (3.93 inches) bigger than the original wood frame.
As you can see, the piece of fabric I used was not an accurate square, but it doesn't matter because it will be cut and hidden at the end.
Lay your piece of fabric upside down on the table and place your seat in the middle of it, with the foam facing the fabric.
Start to staple the 4 cardinal points first, always pulling the fabric, like you did for the foam.
Now continue to attach all sides of the fabric to the wood.
Finally, cut out the excess fabric.
Remember the first square of fabric you attached? Now you have to do the same to cover the hole at the bottom of your seat.
So, you need a square of fabric about 2cm (0.78 inches) bigger than the wood frame.
Place it on the wood.
Now instead of leaving the folding visible at the top like we did before, fold the edges on the inside and start to secure the cardinal points. Then continue to staple the fabric all around the frame, always pulling a little.
Your seat is done! All you have to do now is placing it on your chair...I am sure it's very comfortable! :D