Go to the Cards menu. Tap on Change Deck. You’ll see all your decks there. If you have at least one of the 12 deck slots free, you’ll see a ‘New Deck’ slot. Tap on it to start creating your deck.
First, you will be prompted to select a Warlord. This will determine which Legion cards you can use, and what special abilities you can expect from your Warlord. Tap Select on the top right corner when you have made your decision.
Upon confirming Warlord selection you’ll have a list of 30 cards to fill to the left of the screen. To the right you’ll have a list of all your owned cards available for that Warlord. Drag cards into your deck until you have 30. Tap on any card to see more info about it. You can filter cards by Energy cost tapping on the numbers to the right of the cards, or use the search bar above to search by name or trait.