In order to mark a contract as a Favorite, all you have to do is go to the Contract page and click the star icon as shown in the screenshot below; once selected, the star turns green.
If you are on the Legacy Contract page, this is where the favorite (star) icon is located:
If you are on the New Contract Page, the favorite button is located under the more button (Three dots): 
Afterwards, you can filter by Favorites from the Contracts page, as shown below.
Please reach out to support@contractasafe.com if you need further assistance.