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You will find a clear step-by-step description of -a- in chapter 8 of Dr. Betty Edwards's Drawing on the Right Side of the Brain (Tarcher, 1979). Historically, -b- was derived from the empirical discovery of -a- in the 1430's by Italian Renaissance scientists/artists.
Pedagogically, it makes a lot of sence to introduce perspective in the same developmental order in which it was discovered, starting with visual perception and direct experience, and moving on to the general constructive method, aka "laws of perspective".
Of course this inevitably raises the larger question: is it possible to teach the average student how to draw? Betty Edwards and the people who apply her technology answer yes, but the consensus in most of the art education field is no.
Please let me know if this helps.