what does Santiago do to bring the marlin home?

He attached the marlin to the boat.

"He made the fish fast to bow and stern and to the middle thwart. He was so big it was like lashing a much bigger skiff alongside. He cut a piece of line and tied the fish’s lower jaw against his bill so his mouth would not open and they would sail as cleanly as possible. Then he stepped the mast and, with the stick that was his gaff and with his boom rigged, the patched sail drew, the boat began to move, and half lying in the stern he sailed south-west."

attaches marlin to boat.