Note: this guide is currently under construction.
Piece of Heart #1

In Suthorn Beach there is a water area to the south east with some rock platforms in the middle. Get on a rock platform and then use box and bed echoes to create a bridge to the left where you can find this heart piece on a ledge.
Piece of Heart #2

In the Beach Cave on Suthorn Beach, you'll see this heart piece in a little alcove blocked by three boxes. Get on the platform above these three boxes and drop a boulder echo to break the boxes. You'll have to repeat this process two more times to break them all and clear the way.
Piece of Heart #3

You can buy this heart piece from the Suthorn Town shop for 80 rupees.
Piece of Heart #4

In Suthorn Forest there is a 2D cave to the north. Inside it you'll be attacked by Caromadillos and you'll need to learn their echo to navigate the rest of the cave. At the end of it you'll find this heart piece.
Piece of Heart #5

In Suthorn Prarie there are a grou pof large pillars to the right. You'll notice there is a heart piece on the center pillar in this area. The trick to reaching it is to make use of your new Crawtula echo. Place a bed in the water to the right of the center pillar to give a Crawtula something to crawl over. Then summon a Crawtula echo on it towards the pillar and bind yourself to it. Then follow it as it starts crawling and it will take you to the top of the pillar where you can collect this heart piece.
Piece of Heart #6

In the southern part of Hyrule Field there is a forested alcove that you can enter from the south. There are many enemies in this area, including a Peahat. Defeat them so that you can safely find a heart piece on a stump in the center. You can use a trampoline echo to reach it.
Piece of Heart #7

This heart piece can be found out in the open in the grass on the north side of the outer walls of Hyrule Castle Town.
Piece of Heart #8

This heart piece can be found in the quick sand area north of the oasis in Gerudo Desert. It's standing on a tall pillar, so you'll need to place some echoes like wooden boxes and use them to climb up to it before they sink too far into the sand. This is easier to do after defeating the Lanmola to get rid of the sandstorm.
Piece of Heart #9

In the southwest corner of Gerudo Desert you'll find a group of large sand piles on a cliff. Use a wind-cannon echo to blow them all away to find this heart piece.
Piece of Heart #10

After clearing the rifts in Gerudo Desert, you can climb up on top of the cliff behind Gerudo Town. You'll see a ladder leading down into a cave. Inside you'll need to navigate the cave and then use a Platboom echo to destroy the boxes and bind to the boulder to move it up and out of the way to clear a path to this heart piece.
Piece of Heart #11

There is a cave in the northwest section of Zora Cove. Inside you can solve a puzzle by using a Hydrozol echo to put out the flames of the torches. This will unlock a small room where you can find this heart piece.
Piece of Heart #12

In the southeast corner of Zora Cove there is a cracked rock on the sea floor. Use a Bombfish echo to blast it apart and you'll find this heart piece beneath it.
Piece of Heart #13

You can find this heart piece by diving into the pool of water created by the northernmost waterfall in Jabul Waters. You can use water block towers to reach this higher area.
Piece of Heart #14

This heart piece can be found on a pillar to the right of Lord Jabu-Jabu's Den after fixing the main rift in the area. You can use a tower of water blocks to get to it.
Piece of Heart #15

This heart piece is the reward for completing the middle course flag race at Hyrule Ranch in under 20 seconds.
Piece of Heart #16

This heart piece is the reward for defeating Smog, the boss of the optional Eastern Temple dungeon.
Piece of Heart #17

This heart piece is the reward for defeating Barinade, the boss of the optional Wrecked Ship dungeon.
Piece of Heart #18

This heart piece is dropped by the purple Lanmola found on the east side of Gerudo Desert. It's' the third Lanmola you'll face, as part of the "Wind Sandstorms" side quest.
Piece of Heart #19

There is a ruins area to the west of Hyrule Castle Town in Hyrule Field. You can find this heart piece on a pillar on the east side of the ruins. Use a water block tower or Crawltula echo to reach it.
Piece of Heart #20

On the far west side of Hyrule Field you'll find a pool of water infested with Octoroks. Take out all the enemies and then build a water block or bed tower to reach the tall central platform where you'll find a heart piece.
Piece of Heart #21

There is a cave on the high cliffs to the west of Kakariko Village. Outside of the cave you'll see a bare patch of grass that you can use a Holmill to dig into. This will let you reach the other side of the cave and gather this heart piece.
Piece of Heart #22

There is a large boulder on the high cliffs to the west of Kakariko Village. You'll need to move this up and over the other cliffs to get the heart piece beneath. You can do this by building a water block tower and then binding to the boulder.
Piece of Heart #23

To the east of Kakariko Village in Hyrule Field is a cave with several rolling spools that impede your progress. Rather than using the main entrance of the cave, use a Holmill to dig into the bare spot on the cliff above and you can enter directly into an area where you can pick up this heart piece.
Piece of Heart #24

This heart piece is the reward for clearing the Acorn Gathering minigame to the northeast of Kakariko Village in less than 25 seconds.
Piece of Heart #25

This heart piece is the reward for completing two trainings in the Slumber Dojo in Kakariko Village.
Piece of Heart #26

Near the southwestern waypoint on Eldin Volcano Trail there is a short rock wall with a patrolling torch slug above. Bind to this slug and let it carry you over to a small ledge to the left where you'll find this heart piece.
Piece of Heart #27

This heart piece can be found on a tall cliff between Rock-Roast Quarry and Lizalfos Burrow in Eldin Volcano. You can build a water block tower to reach it.
Piece of Heart #28

This heart piece can be found on a small platform in the lava to the east of Lizalfos Burrow on Eldin Volcano. You can get on the cliff to the right and then use a water block or bed bridge to reach it.
Piece of Heart #29

This heart piece is the reward for beating the Flame Giant boss at the end of the Summit Cave mini-dungeon in Eldin Volcano.
Piece of Heart #30

After creating the Gizmol automaton with Dampe, speak to the Zol kid on the west side of Hyrule Castle Town to get the "An Out-There Zol" side quest. Shom him the Gizmol to get this heart piece as a reward.
Piece of Heart #31

During the Deku Scrub Lockup in Faron Wetlands you can find a hidden room by moving a shrub away from the wall. Inside you'll find this heart piece on a totem pole. You can rearrange the other items in the room to build a ladder, or wait until you have your echoes back to get up easier.
Piece of Heart #32

In the southeast of Faron Wetlands, west of the Hidden Ruins there is a cave entrance. Inside you can build a bridge to a platform to the left where you'll find this heart piece.
Piece of Heart #33

There is a cave on the west side of Faron Wetlands that leads to an underwater maze. Navigate this maze until you reach an open area with dragonflies above. Take them out and build a water block tower to reach a ledge to the left. Defeat the Armos (get on the other side with a trampoline) and you'll find this heart piece to the left.
Piece of Heart #34

Given as a reward for beating Smog, the boss of the Hidden Ruins in the Faron Wetlands. This is done as part of the "Cotton-Candy Hunt" side quest.
Piece of Heart #35

This heart piece can be found on a tall cliff on the north side of Hebra Mountain. You can climb the cliff as soon as you emerge from the Hebra Mountain Cave exit.
Piece of Heart #36

This heart piece can be found on an icy platform in the southwest of the pool of water on northern Hebra Mountain.
Piece of Heart #37

Along the snowy peaks of Hebra Mountain where the snowballs are rolling down the mountain, there is a platform to the northwest that you can jump to and find this heart piece.
Piece of Heart #38

This heart piece is a reward for completing the "Snowball Magic" side quest given by Condé in his home after clearing the major rift on Hebra Mountain.