421.1. Decks must be made random at the start of each game, and whenever an instruction from cards or tournament officials require it.
421.2. Random is defined as a state where no player could have any information about the order or position of cards in any part of the deck.
421.3. Shuffling by methodically placing cards into distinct piles (informally known as “pile shuffling”) may be used as a method to count the deck, but can’t solely be used to make a deck random.
421.4. When a deck is made random, it is presented to the player’s opponent. This signals that the player believes their deck is legal and random.
421.5. Players must notify a judge if they believe their opponents’ deck has not been made sufficiently random. See 703.7 for more information on insufficient randomization.
421.6. Players may shuffle their opponents’ decks additionally.
421.7. Players must take care not to damage cards or sleeves through this process of shuffling, nor cause cards or sleeves to become marked.
421.8. When two or more cards are recycled to your Main Deck as part of a single action, the above procedures 1-7 should be followed for that group of cards to properly randomize them before recycling them.