In a written Canvas — an essay, a chapter, an article — Echoes attach to a paragraph.
Long-press a paragraph on mobile, or right-click on desktop. The Echo picker appears with the small set of reactions. Pick one.
The paragraph is what gets remembered, not the line number. That matters because line numbers shift around based on font size and screen width — your "line 12" is someone else's "line 18". Anchoring to the paragraph text means your Echo always points at the right thing, no matter what device the creator opens.
You can leave one Echo per paragraph. Leaving another on the same paragraph replaces the first.