I say 'nothing much', but a major part of the deal is that you spend the time scanning your garden for the occasional flash of feather and beak. If, like me, you believe a garden's primary purpose is as a playground for birds, you'll consider this time well spent. The only actual work involved is the putting of pen to paper, and not a great deal of that in truth.
Register on the website, and download one of the handy 'counting sheets' to help you record your count (I notice Chickens are not included... Bah!). It's not a competition, but should you decide to carpet your garden with tasty, beak-sized morsels in the hope of attracting more feathery activity than your neighbours... well I don't suppose it's against the rules.