Passport makes it easy to add games you've already attended as well as ones you're planning to attend. To start, go to the Add Events page. Here you'll find a list of all the major sports teams along with a tab for your own favorite teams.
Click on the team you're looking for and a calendar will appear, allowing you to select one or more games. Once you've found all the games you want to add, click "Continue" to rate your experience at those games.