Read the screenplay, visualizing how it looks in your imagination. As you do, block off each section of dialogue that you imagine as its own scene. Write notes in the margins describing what you see, if it differs from any visual instructions that are already written in the body of the script.
Take out a sheet of paper for each blocked-off scene. Write a short description of each scene at the bottom of the sheets.
Sketch each scene as you visualize it, adhering to the description that you've written. Mark movements of the camera with arrows -- pointing right or left for a pan, up or down for a tilt, and inward or outward from the corners for a zoom in or zoom out, respectively.