You may mill two cards. Then return up to two creature cards from your graveyard to your hand. (To mill two cards, put the top two cards of your library into your graveyard.)
If you don't have at least two cards in your library, you can't choose to mill two cards.
The creature cards you return to your hand don't have to be any of the cards you milled. They can be cards that were already in your graveyard before you milled.