Rapidly draw horizontal and vertical lines on the paper, varying line length and intersections between the lines. At a further stage, begin to draw lines from line ends and crossings roughly towards the centre of the paper, or outward, again using different line lengths. Avoid drawing a specific vanishing point.
Do a number of these exercises, changing the speed of drawing and the content of the exercise. A two-point perspective can also be used, receding the horizontal lines to an imaginary point outside the paper.
The already existing lines on the paper guide the following lines, but this should not be rationalized too far.
1. Draw interior space and form as naturally as objects from the outside
2. Unlearn the idea that drawing an interior means establishing a rigorous perspective method
3. Develop ways to seize opportunities from the drawing and to alter the exercise itself