Take the first 13 cards off the top of the deck and flip them over. This is the first pile. The second pile is just the remaining 39 cards as they started.

The logic can be a bit difficult to digest, so let’s look at an example. Let’s assume that from the first 13 cards, 5 of them are face-up. So from the remaining 39 cards, 8 cards will be face-up. When we flip the cards from the first pile of 13 cards, 8 of them will become face-up and 5 will be face-down.