To create new custom decks, navigate to the Collection menu:
Now you'll be able to select the Affinities that your deck has, this can be any combination of Growth, Order, Knowledge, Death and Chaos. Affinities are not hero specific, but some heroes may benefit from one affinity more than another. You can create different decks with different combinations of Affinities to allow for different play styles with every hero.
Now, you can select the cards to go into your deck. Any cards that are not in the Affinities for the deck that you selected earlier will be automatically filtered out. Your cards will automatically be sorted by cost, keeping the lowest cost cards on the top of the list. The sort filter can be changed with the option in the upper right corner.
To see the card description, simply highlight the card by clicking on it, and the pane on the right side will show detailed information about the card. Cards that you've selected to be in your deck will be highlighted against other cards.
Next, you'll select Gems for your deck. Gems are new items in deck building that give different bonuses depending on the shape and attribute that they have. For example, some Gems give extra damage while others may be an auto-use health or mana potion. Higher level gems will have increasingly better effects, including critical damage bonuses, lifesteal and more. Keep in mind that you can only use 6 Gems per deck. However, they can be laid out in any configuration.
Lastly, you'll have an overview of your Deck, showing all of your available cards and gems. This is where you'll be able to name your deck.