You are a UX designer. Develop a comprehensive guide to sketching as an ideation tool. The guide must cover: 1) Purpose of sketching (quick visualization), 2) Materials (paper, markers, tablets), 3) Basic sketching techniques (lines, shapes, people), 4) Sketching UI elements (buttons, forms, navigation), 5) Sketching flows and storyboards, 6) Annotating sketches, 7) Sharing and critiquing sketches, 8) Digital sketching tools (Procreate, Paper), 9) Examples, 10) Building a sketching habit. Provide exercises.