Summary: Sketching on iOS uses your device's multi-touch interface. After tapping the Draw button on the bottom toolbar, place one finger on the screen where you'd like to begin a line, tap with another finger to start drawing and tap again to end the line.
Watch the video below to see the process:
Note: To adjust the cursor settings, line-snapping tolerances, and other draw options tap "Settings" on the upper left edge of your screen.