All over the world of "The Legend of Zelda: Wind Waker" you will run into a common enemy, the Moblin, which drops the Skull Necklace. After you obtain these necklaces you will be able to find a couple of individuals that will be willing to trade you for all your hard work.
Obtaining the Skull Necklaces
There are only three ways you can obtain the Skull Necklace. Either you have to defeat a Moblin and take one after you win the battle, steal one from a Moblin with the Grappling Hook or find one in a treasure chest.
Atter you collect at least 20 Skull Necklaces, travel to Windfall Island, which you find in D2 on the world map. Find Maggie's Father in the Hall of Wealth and trade him 20 Skull Necklaces. In exchange, he will give you a Treasure Chart. You'll need to have rescued Maggie before being able to make this exchange.
Trading with Dampa
If you don't have 20 Skull Necklaces to trade, you can go find Dampa, who is standing outside the Bomb Shop on Windfall Island. Talk to him to trade three Skull Necklaces for a chance to play his pig-finding mini game. If you're playing the HD version of the game, you will get a Treasure Chart if you beat his game.