You do, of course, need iOS 6, which is currently only available as a developer release. But, once you have that, the rest is straigtforward. As 9to5mac explains:
First, open the PassSource.com website on your iOS 6-enabled device, and then select which type of pass you would like to create.
Fill in the pass with the correct information, like the card number and member name.
It will then open inside of the Passbook app, which will make the app completely usable!