How did you snap those clips (the ones that kinda look like an omega shape) on to the upholstery material? They are much too small to snap in with fingers and pliers will ruin the material.
We used to have a special set of pliers with rubber coated jaws for clips like that. For as few clips as there are on a single seat, you could probably get away with wrapping the jaws of your pliers in 5-6 layers of electrical tape to protect the fabric. Or if you wanted to be really fancy, grind the teeth smooth and dip it in that rubber stuff for making new handles on tools.