You're gonna layer your rolled out butter now into the dough. I roll out two sticks (1 cup) of almost room temperature butter between two sheets of waxed paper, stick it back in the fridge, until the dough is ready for you. Remove one layer of waxed paper, place that side down on the dough. Take off the last layer of waxed paper.
Fold this in thirds. Flatten out and fold in thirds again. So you've done it three times. Which is great, because you want layers of butter to make your croissants flaky.