Heart Piece 1
Give Batreaux 10 Gratitude Crystals.
Heart Piece 2
Upon obtaining the Medium Wallet and two or more Extra Wallets, or the Big Wallet (the Medium/Big Wallets are Gratitude Crystal quest awards, and the Extra Wallets are bought from Beedle), in addition to the Slingshot, go to Beedle’s Airshop. Buy the Piece of Heart for 1,600 Rupees there. You will need to buy the Life Medal, first, however.
Heart Piece 3
To do this one, you’ll need to have finished Fledge’s sidequest for the Gratitude Crystals. Once you have, speak with him outside of the Sparring Hall and play his Pumpkin Pull minigame. Once you obtain 600+ points, you’ll get the Piece of Heart.
Heart Piece 4
For this one, you’ll need the Water Dragon’s Scale and Goddess Cube 15 activated – it’s on the large slide at Eldin Volcano, using the first air vent on the left.
Drop into the river east of the Bazaar and swim through the hole and tunnel beneath Kukiel’s house. Upon emerging, you’ll find two chests. The Goddess Cube chest contains the Piece of Heart.
Heart Piece 5
After obtaining the Clawshots, go to the rooftop of the Knight Academy. Look above the chimney to find a target; Clawshot there and go into the chimney. Crawl through the tunnel to find Zelda’s room. Examine her dresser to find the Piece of Heart.
Heart Piece 6
For this, you’ll need to have the Clawshots. You’ll also need to have activated Goddess Cube 26 in the Pirate Stronghold of the Lanayru Desert; it is found atop the ledges behind the door you exited through the first time.
The chest is found atop the waterfall. It contains the Piece of Heart inside it.