Perhaps some of you remember that when I go and visit special people, I like to clean their garage. Here's a treasure from THIS organizing adventure. It made its way from Minnesota to Illinois, and then ended up here.
First off, I cut samples of what I thought the cushions would be, and then held them up to the real chair. Once they looked about the right size, I cut the actual piece from the upholstery fabric. I also made welting from a contrasting fabric.
To make the cushions, I made fully filled pillow forms that were the same shape as the pillows. Making these pillows with a zipper is the best way to go. If things "shift" over time, they can be adjusted easily. Once you squeeze the forms in, I shove more stuffing in the nooks and crannies of the pillow form. It makes it look sort of "pregnant", and that's a happy look for upholstery.