simply a matter of seeing how it was done at the factory, carfully cutting out the original stiches. Once the original fabric was seperated that became our patterns for the new fabric. Cut the new fabric to the same shapes and stitch it back together
True...unfortunately, we washed our covers to get all the disintegrated foam off and they shrank & fell apart. If you are going to make your own, you may need to make an intermediate pattern to keep from soiling your new fabric.
Also keep in mind that if your fabric is heavier or lighter than the "original" it will fit differently. There is also the matter of getting heavy duty 72"+ zippers. The pros have zippers in a spool & a cool machine that puts it all together perfectly in whatever length is needed.
I was fortunate (& picky), my seamstress let me help/watch....and she didn't even seem bothered by me, nor did she charge me extra.