You start with a piece of paper card cut roughly to size and shape. I used a piece from an old Business Card which seems perfect. Once you have the correct cut-out, glue it in place with super-glue, allow to dry then go back and add glue to the entire surface area of the card back and front creating a plastic-card type surface (which means making the paper harder so you can work with it).
Once that is nice and dry you can go ahead and add your greenstuff and sculpt your fur pelt. See my Fur Test post for an idea on how to sculpt fur.
Pretty easy to do so don't be scared to try it out.