To attempt this, you must have a Cannon and a Salvage Arm on your ship. Head to the southwest section of the map, near Mercay Island. From here, you'll want to head towards Goron Island, where the pirates seem to congregate. You can destroy the pirate vessels with your Cannon, and then use your Salvage Arm to take random parts from their sinking ship. You can do this over and over again, grabbing parts like this instead of purchasing them with your hard-earned Rupees.