There's some YouTube about that.
Here's some great pictures and instructions that may be helpful:
https://gallery.rennlist.com/gallery/album651
If it's like my seat, the material under the cushion came apart. I was going to post a How-to, but never got around to it. Here are some pics of what I did:
The Problem:
Remove the bad, cut a better replacement:
Insert zip-ties:
'Sew' it together:
Reassemble the seat:
