I'm not John either, but some days I wish I had his skills . . .
Anyway, it looks like @Daniel u has not been back in a few weeks, but it terms of 'creases' . . . those are more commonly referred to as 'wrinkles' . . .
TENSION is your friend. The fabric/skin is probably a bit too big or has not been pulled enough on the back side of the cushion. Through my experience doing boat seats, I have found that the vinyl 'skin' needs to be 2-3% smaller than the cushion so that it needs to stretch to fit.
If the vinyl skin is the exact same size as the cushion/foam, then there will be little/no tension on the vinyl . . . in order to get tension you will need to pull on the back side, but in doing so, you will tend to pull the skin out of its intended alignment with the foam and base board.
Inside curves are the hardest to do (which I think is what's shown in Daniel's original post) . . . much of the tension needs to be up/down in the fabric to hols it to the foam on the inside curve as opposed to side-to-side, which will pull the fabric away from the foam.