G-Codes to Stepper Pulses?

I'm thinking of hooking up a Smithy Granite for CNC using stepper
motors. I have a book that shows how to drive the stepper from a PC but
I don't know how to convert the G-Code output of various packages like
BobCad or FlashCut into stepper pulse commands. I am a programmer so I
should be able to do it myself with a detailed definition of G-Codes and
maybe a few tips. Where can I find an unabridged G-Code definition?

Any other advice? I would like the cheapest most accurate system
possible, capable of working small stainless parts to 0.001" or better.

Thanks,
Kirk Fraser