The speaker is a beginner at drawing, and the lesson is mostly about looking. Once she tries to put things on paper, everything she thought she knew turns out to be wrong. A cat becomes an oval with fuzzy edges. A dog's tail looks like a beret sitting on her head. A book is only as wide as its spine, because that's all you can see from the side. Trees shrink, roads disappear, and shadows melt into teacups.
The tone is curious and a little amazed, with some quiet humor in the beret image. The speaker doesn't sound frustrated. She sounds like someone discovering that the world is made of shapes, edges, and darkness rather than the neat objects we name. "Nothing squares" captures this well: real things refuse to fit our tidy ideas. The last line, about windows being the darkest parts, surprises us, since we expect windows to be bright. Drawing has taught her to notice what's actually there.