Place one finger on the screen. Hold for a moment and slide your finger in any direction to start drawing. While you are drawing, you will see the angle and the length of the walls. Do not worry if the measurements are not very accurate - you can edit the walls afterwards. To stop drawing, simply remove your finger from the screen.
Adding walls works the same way - hold your finger anywhere on the canvas and hold for a moment to start drawing again.
Watch our video tutorial: