Click Submit Work on /critique, or go straight to /critique/new. Add your images, a category, and context on what you're working on and what kind of feedback you're after — the more specific your ask ("is the hierarchy on this pricing table clear?" beats "thoughts?"), the more useful the responses tend to be.
Once it's live, you'll find it under the My Critiques tab on /critique, and you can come back to reply to feedback as it comes in.