Approximately three-fourths of the way through the second level (on your way to save General Kota just after the fight with the Wardroid), Starkiller will encounter a hologram of Jabba The Hutt, and there will be a lot of golden Guybrush Threepwood statues all around. Destroy the three slot machines directly in front of this hologram, and a red holocron will appear. Pick it up to unlock the Guybrush Threepkiller costume. Pause the game, and select the "Costumes" option to equip it.