This is a fairly difficult exercise to master, but it can be quite rewarding if taken seriously.. begin with a clear mind, at least be comfortable, and thinking mainly of your breathing. Now, imagine a light with color of your choice, shining like a star through the darkness. Now imagine this light stretching like a beam, slowly making sharp turns until it creates a shining square( or triangle). Now turn this image on every direction you can think of, upside-down, spinning on each corner individually. Once this becomes easy for you, try to expand the two-dimensional shape into its three-dimensional counterpart.. remember holding the entire shape in your "vision" is the key here. Later you can try filling in the shape, and collaborating two different shapes together.