Hearts come later in the game. (You probably noticed the heart graphic on the pause screen.) There's no point trying to get them early, and in most cases it's impossible since the sections they're in are blocked by obstacles you can't get past yet. If you aren't being sent for hearts yet, ignore this guide for now. Talk to a Fortune Teller whenever you aren't sure where to go next.

A woman in Stemmonds Community gives hints for the heart locations. She's in the second house, wearing a yellow dress. Her hints are outside of spoiler tags, since the game gives you them anyway. Additional hints are in tags.

Six pieces of the King's Heart: One each in the South, East, and the West. Two in the North. One deep underground.

That's her overview hint, mostly redundant to the more specific hints she gives. It can help with "process of elimination". If you can't figure out a clue, and have 5/6 hearts, at least you'll have a general idea which direction the last is.

You may not need any of the extra help here; just having the six game hints in a handy list may be plenty of help. If you look at hints, watch out that they get increasingly spoilery as you go down. The first each in section, labeled "Basic", is vague and encouraging, not really a spoiler. But the second-last will outright tell you the answer. And the last is a link to BlaXpirit's map. So be careful and only read as far as it takes.

I felt a heartbeat below a pool of lava in Lakewood Dungeon.

• Basic: This counts as East. Explore the dungeon looking for a "pool of lava".
  
• Hint: There's only one area of lava in the dungeon, and it leads to the heart. If you see red lava on the floor, you're there.
  
• Hint: At the start of the dungeon, there's a main left/right fork after jumping down. Go left for lava.
  
• Hint: You'll need to go swimming in the lava. The lava-free hole through the middle of the room isn't it.
  
• Spoiler: Jump in the left side of the lava, and keep going down.
  
• Spoiler: Map[blaxpirit.pythonanywhere.com]

I felt a heartbeat within a hidden nook near the Great Hall in the Desert Ruins.

• Basic: This counts as South. Talk to the npcs in the Desert Ruins; one will let you know which room is called the Great Hall.
  
• Hint: The Great Hall is the big room with the Subcon door, and the moving platform that you steer by jumping on it to change its moving direction.
  
• Hint: You already found one "hidden nook" in the Great Hall, the first time you got through it, by jumping through a spike wall. Look for another.
  
• Spoiler: It's right by Subcon, in the top-right side of the Great Hall. Leaving Subcon, go straight right and jump through the wall.
  
• Spoiler: Yo dawg! The evil wizard heard you like hidden nooks. So he put a hidden nook in your hidden nook. So you can find hearts while you find hearts.
  
• Spoiler: Fall into the pit towards the left side, and use the tiny platform to jump left through the wall.
  
• Spoiler: Map[blaxpirit.pythonanywhere.com]

I felt a heartbeat in the ground beneath the Sky Pillars.

• Basic: This counts as West. The hint is fairly straightforward. Read it again and you might see what to try.
  
• Spoiler: Jump down at the "base" of the Sky Pillars -- the bottom of the first screen -- into a hidden basement level.
  
• Spoiler: Map[blaxpirit.pythonanywhere.com]

I felt a heartbeat within a cave in the Snowstorm.

• Basic: This counts as North. The Snowstorm is the big area in the overworld that looks like static.
  
• Hint: Of course what you're looking for will be towards the center, where things are hardest to see...
  
• Hint: See some specs of green?
  
• Spoiler: Get on the north side of the green part, and walk south into it to enter.
  
• Spoiler: Map[blaxpirit.pythonanywhere.com]

I felt a heartbeat beneath a lake of icy water.

• Basic: This is probably the trickiest one to find. So don't feel like you're "missing something obvious" and need a hint. If you want to keep trying, keep trying.
  
• Hint: It's North.
  
• Hint: There are a lot of little passages north, but you want one that's a "lake of icy water". There aren't a lot of those. If you think you've found one, it's probably it.
  
• Hint: Make sure to look beneath the lake, like in the hint.
  
• Hint: It's the bridge area connecting to the Glacial Palace.
  
• Hint: You probably found a door in the bottom-left corner, leading to a fish and a gem. That's not where the heart is. Look around different parts of the lake.
  
• Spoiler: There's a fall-through section in the bottom of the lake. Some of the spikes are fake.
  
• Spoiler: From the save bell at the middle of the area, jump about 2 tiles right, and fall straight down.
  
• Spoiler: Map[blaxpirit.pythonanywhere.com]

I felt a heartbeat among the Subterranean Waterways.

• Basic: This is the "deep underground" one. There are a lot of entrances to Subterranean Waterways! But it's all connected. Enter anywhere, and explore until you find the area with the heart.
  
• Hint: Look for sections you couldn't explore on your first visit, ones blocked by toxic water.
  
• Hint: Towards the left end of the area.
  
• Hint: The bridge by Lakewood (between Stemmonds Community and Lakewood Dungeon) is a good starting point. Jump in the water, and go right for an entrance to Subterranean Waterways.
  
• Spoiler: Once inside, you'll see a sign and a bell. Jump in the toxic water below the bell, and head left. And keep going; it's not short.
  
• Spoiler: If you think you found a dead end, try falling through some floors.
  
• Spoiler: You'll find one blob moving vertically by itself, a lightning wall, then 3 blobs moving together. Fall down beneath the single blob.
  
• Spoiler: Map[blaxpirit.pythonanywhere.com]

Since I'm encouraging you to do this mostly yourself without answers, here's a little list you can copy and check off.