The neat thing about surface plates is that they do not require precision tools to create. By using the “three plate method”, developed by Joseph Whitworth, flat surfaces can be created by using gravity and a simple hand-scraping tool, or by lapping the plates against each other. By starting with three plates of relative flatness, rubbing the plates against each other in alternating pairs to remove the high spots can yield fantastic results.
The process can be completed in six steps, and then repeated until the desired level of flatness is achieved. In this visual explanation, the surface finishes of the three plates are exaggerated. Before beginning this process, the three plates (Red, Green and Blue) should be machined or ground to as flat a surface as possible, to remove all unnecessary lapping work. In addition, a fine abrasive compound is often used between plates to assist in material removal. Tom also has a great video series about this, which includes some compound and technique recommendations."