The way the upholstry is attached to the seat spring frame is harder to explain than it would be to show you. The edge of the upholstry that "attaches" to the spring frame has like a cardboard edge material that forms a stiff straight section along its edges where it attaches to the frame. This edge piece "hooks" into a u shaped channel in the edge of the spring frame work. You have to sit on the seat and this provides enough slack to allow you to pull this edge out from the u shaped channel. Once you do one it is pretty straight forward. I will say that the last couple I did I had to use a screw driver to help pry along some of this edge section because after being in there for almost 20 years it was pretty tight. If someone can explain it better please do. Unfortunately, I do not have any pictures of this job.
Again, I stongly recommend replacing the complete spring frame assembly with a new one. The spring assemblies are available new and are not cheap but you are wasting your time getting one from the pull a parts place. The one I took out of the 300SD was not busted or broken but it went into the trash can. I refuse to sell something like that on ebay when the newer design is 100% better.