Use your measuring tool to find out how much fabric you will need approximately for the size of your back and seat pad. You'll need enough to cover the edges, in order to staple it on the back of the pad, later on. I recommend using it double if it's thin fabric.
Before you span the fabric over the pads, think of a cool print for the backing pad. I don't recommend having an illustrated seat, for you will have to sit on it. And if you do that you show the design that you do not respect it. Now, iron your cloth and use this method
to get a stenciled piece of fabric. If you are in possession of freezer paper, use bignothing's method