Theseus snuck up behind the bull while it was drinking water and grabbed it by the horns. He then tied the bull up, making sure it could not escape, after wrestling with it.
He found the bull drinking from a water spring, and snuck up behind the bull. Grabbing the bull by the horns, He wrestled the bull to the ground and tied the bull up so it could not escape. With the bull successfully subdue, He took the bull with him and sailed back to King Eurystheus.