In after you go past Lake Hylia and go to the top of a waterfall, you reach the Faron Wetlands.

Explore the Faron Wetlands

From the waypoint at the northwest entrance of the Faron Wetlands, go east from the soldier, then south, and defeat the blue Zol and learn the Hydrozol echo.

Go south from there, and there will be a cutscene.

Go east and defeat the singing frog and learn the Drippitune echo.

Then go east and south and defeat the creature with glowing mushrooms on its back, and learn the Hoarder echo.

To the east, you can talk to a Deku Scrub, then you can bind to the scrub next to it and pull it out to talk to it as well.

In the northeast part of the area, there will be a cutscene. You can then go east from there to find some new monsters and learn the Baby Gohma echoand the Buzz Blob echo.

Make your way southwest to the Smoothie Shop. West of the shop, there will be a cutscene. Farther southwest, there is a scrub who tells you to check the heart-shaped lake to the southwest, because someone there might have a membership card.

On the way, look for a cave. Inside, you find Armos underwater. Bind to the Armos and pull on it to turn it around, exposing its unprotected back. Then use Bombfish or swordfighter form to hit its back and defeat it, and learn the Armos echo.

When you reach an area of the cave with Needleflies, create water blocks above you and go to the surface to find a Piece of Heart on the left.

Make your way south to where the heart-shaped lake is shown on the map. When you arrive, check on the waypoint, then approach the scrub in the water and there will be a cutscene. Check on the glowing spot in the rift in the lake. You'll arrive in Stilled Heart Lake.