Percy and Grover travelled back to New York on the Greyhound bus. When Grove was telling Percy about the conversation he had had with Mr. Brunner, the bus broke down. At that moment, Percy saw three old ladies knitting sock on the side of the road. These old ladies were performing a kind of ritual when Percy saw one of them taking out a pair of scissors and cutting a piece of yarn. Percy tolf Grover what he had seen and guessed that someone was going to die. As a result, Grover became terrified.
If you mean in the movie, The Lightning Theif, then I think I can help! The traveled back to N.Y.C. by using these three little pearls that could only be obtained by defeating three monsters: Medusa, a two(or three) headed beast that I forgot the name, and Ares (the Greek God of War). Percy, Percy's mother, and Annabeth used those three pearls to teleport back to New York City (or just N.Y.C.)
