To use the Deckbuilder, download the version that corresponds to your computer's operating system, and download the Starfall json file.
When you use the application for the first time, it will prompt you for the location of the Starfall json file. Select it and it will automatically construct all the cards from the set for you.
Simply click a card from the right side to add it to your deck, or right-click a card to add it to your sideboard. Click a card from your deck view on the left side to remove that card.