Tap Shapes, then choose Box, Circle, Line, Arrow or Text box and drag on the page. Only one shape is drawn at a time: tap Done, then choose the next one.
To change colour tap a colour circle in the panel. Fill colours the inside, and Thin / Medium / Thick changes the line width.
Made a mistake? Tap ↶ at the top to undo (↷ to redo). Nothing changes your original file until you download.