The Short Version: Pick a card from the deck, and do what it says. If you can’t, drink.
The Long Version:
Shuffle the Truth and Dare decks, keeping the Blackout deck separate.
The group takes turns drawing cards, going in a circle.
Each person reads the prompt out loud, and chooses whether to follow the instructions or take the penalty sips.
The group draws a Blackout card after each round, and follows the prompt together.
Continue playing until the cards are done, or the group is sufficiently quenched.