Once you sign up, you go to the list of shops. There will be about 185 places. Click on the link to a shop, then explore in that site until you find the bunny. Click on the bunny (the one that says "You found it") and it will take you back to the fab shop hop site where you enter your e-mail address to register for prizes. Just keep doing that over and over until you've visited all the shops.
I have only once found all the bunnies and have never won anything. Some sites can be frustrating to navigate and others are nice and straightforward. It's interesting to see all the on-line places, though.