You need to turn it over remove the bottom cover, usually black and stapled on rail. Carefully remove the staples or tacs holding the upholstery. Should be able to pull up enough to get to top of seat so you can add to or replace fill and foam. Put it all back.
Thanks! Sounds simple enough but when I actually paid attention to the number of staples I chickened out (for now). I'm going to do a practice run on a chair that I use at a desk first.....
You can buy the foam in several thickness. Just cut with electric knife the same as the one that was in there. U can use the fabric to make a pattern also but harder to get it correct
Make sure the foam is soft enough to sit on. I bought several pieces of foam for my couch seats and had to take them back because they were as hard as rocks. The store said they were really made for boats!
Wow - thanks for the heads up - great to know!