At some point you're going to get a piece of chalk. This guide is for people struggling with what to do with it. Please free on only if you're having difficulties specifically with the chalk item.

....................................................... Potential Spoilers Below....................................................
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Ok, so you're at the tree, you have a piece of chalk, and you know what to do with it, but can't seem to get it to work on that cloud in the lower left.

I struggled with all sorts of shapes and attempts to draw that last cloud. What worked for me was drawing a obtuse triangle, with each angle from the base/hypotenuse being about 30* (degrees)

I'd start by trying that next if you're reading this. Additional things I noticed that definitely didn't help included:
- Don't attempt to fill in the shape.
- Don't attempt to follow the outline that closely.
- Don't attempt to match the shape of the arrow on the right side.

-Do try to draw just a three sided obtuse triangle shape.

TLDR: Draw a triangle that fits within the cloud outline.