Use the left analog stick to move Ratatouille over to the tightrope or other narrow surface that requires you to balance while crossing.
Press and hold the "Square" button.
Push forward on the left directional stick to move Ratatouille across the narrow surface. Jump with the "X" button while continuing to hold the "Square" button. Ratatouille will balance as he crosses the narrow surface and make it to the other side.