The basic gameplay is to eat mushrooms with your starting animal and use the mushroom notes to sing along with the boulder. Songs are marked with ♫.
Just keep going right.
All achievements are storyline.

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Click on "Song Animals" to start the game.
Eat Mushroom #1.
Click on the boulder and then click on your mushroom (while the boulder is singing).
♫ 1

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♫ 1
Use Flappy (animal #2) to knock down the tree.
Eat Mushroom #2.
♫ 121

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Use Bitey (animal #3) to knock the boulder down.
♫ 21
Eat Mushroom #3.
♫ 231

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♫ 31
Use Flappy to get Mushroom #4 and eat it.
♫ 421
Use Bitey to knock down the vine.

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♫ 341
Use Mousey (animal #4) to get Mushroom #5 from the small tunnel and eat it.
♫ 2534

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♫ 3421
♫ 12345