For each attacking creature, its owner puts it on the top or bottom of their library.
7/18/2014 The owner of each attacking creature chooses whether to put it on the top or bottom of their library. The active player (the player whose turn it is) makes all of their choices first, followed by each other player in turn order.
7/18/2014 If an effect puts two or more cards on the top or bottom of a library at the same time, the owner of those cards may arrange them in any order. That library’s owner doesn’t reveal the order in which the cards go into their library.