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.