Recycling is done in the Crafting section of your Collection Page. Recycling will get rid of all your extra cards, as you can only have 2 of the same card in your deck, so you will at most only need 2 of a certain card. This excludes your Diamond cards, as you only need 1 Diamond card, as you can only one 1 Diamond card per deck. Other info on Recycling can be found on the Crafting page under Card Rarity and Gem Exchange.