When you are describing,
A shape, or sound, or tint;
Don't state the matter plainly,
But put it in a hint;
And learn to look at all things,
With a sort of mental squint.

The stanza playfully advocates indirection in description: instead of stating “the matter plainly,” the writer should suggest, imply, and let the reader complete the picture. Carroll’s comic phrasing (“a sort of mental squint”) turns a serious aesthetic principle—obliquity, metaphor, and the power of implication—into a joke about deliberately looking at things askew. Read in the context of Victorian nonsense and parody, it also gently mocks didactic rules for “good writing,” implying that technique can become a mannerism. The lines celebrate imaginative perception: to see freshly, one may need to distort ordinary angles and rely on hints rather than literal statement.