Master A Deck
From this page you can set a deck as mastered and get the rewards for it.
The first drop down gives you a list of decks that are able to be mastered. In order to master, you need to have at least 1 of each of the cards.
The second drop down is the available master badges, select one for it to load the image. Select the one that you want to use.
The next set of drop downs allows you to select the choice cards you get. You can either select the deck and card manually, or select the collecting deck link which will automatically set them for you.
If you are mastering the deck you currently have set as your collecting deck, you can use the final drop down to change this deck.
Once you are happy, select update and it will present you with your cards.