The keys aren't too hard to find, but there's a whole world to hunt around and sometimes the clue isn’t so specific about which level to go to. As a last resort, this page will tell you where to go for the keys.
World 1: Koopa Plains
“Between two hills in a level at the bottom of the world is a block you cannot see”
Well, there’s three levels that fit that description, but it’s the middle level of the bottom row that has the key. You have to know to hold Down for a while on the white block to get through. A little while after the music note block pit, you’ll see two groups of hills with eyes on them in the background. In between them is a music block. Ride it to heaven. Finish the level (or die, it doesn’t matter), and reset the game to use your key.
World 2: Hot Foot Caverns
“A fire spitting boo is guarding the entrance which lies near the end of the world and the level.”
It’s the right-most level, and yep, it’s near the end of it. When you get to the Fire Boo, jump to the lowest wood platform. The left side of that has an invisible music note block. Unless you’re Fire Fire Mario, hop onto a higher platform before attempting to jump on the music note block.
You can see where the invisible blocks are, because no rain will be falling through them. When you see the Fire Boo, look for the one spot that is clear of rain.
World 3: Lakitu Glacier
“Under the Bottom of the ocean is the key to finding the key”
It’s pretty vague. Looking at the map, you’d probably go for the lower levels since they look like they’re underwater, but no, this ocean is actually above water. It’s the fourth level on the path.
When you see the chain chomp, swim down and to the right. There’s enough space to swim under the blocks. Move up the upside-down pipe to get the key.
World 4: Kuribo’s Woods
“The right of the cloud gets the key... at least it did in the past!”
The “past” past of the clue refers to the level one step left of the fortress. That level is based on World 1-1 from SMB1. A music note block appears just right of the first cloud, right at the start of the level. Now you get to play a new version of the level filled with Brothers, and the Boom-Boom is replaced by a key.
World 5. Starry Slopes
“The arrow points to the coins needed to pay the toll to see the door”
OK, so hopefully you remembered where there was an arrow in this world. Well I’ll just tell you; it was the fortress. Like the World 4 clue, this clue isn’t really accurate. The fact is, the arrow is made of the coins needed to pay the toll to see the door.
Anyway, start the fortress as normal, but make sure you collect every coin. When you get to the arrow, collect all those coins, but instead of dropping down, go back to the start of the level. The dropping music note blocks have a door on them now. Enter the door, pressing up as soon as you land on the block before it drops. Congrats, that’s key #5.
World 6: Colossal Classics
“In the Land of Giants, a tiny way is for tiny people where music notes are silent and fake”
Look at the fortress, now take the low route to the left of that, and play the first level you run into. When you get to the part with the music notes, get shrunk into little Mario if you’re not already. Now go on the ? block and hop. The top row of music notes are fake. You get a key made out of coins, then go down a pipe so you can get the real key.
World 7: Desert Dares
“Toot!” (letter comes with a whistle)
Toot the whistle.