This work describes the use of Monte Carlo simulations to develop a set of gamma ray selection criteria for images of Cherenkov light in air showers, taken using the Whipple Gamma Ray Collaboration's 10-meter atmospheric Cherenkov detector. These selection criteria are suitable for the extraction of the energy spectra of the observed sources, and have been applied to the Crab nebula. I describe the method for the spectrum extraction and discuss possible systematic biases in the derived spectra, arising from an energy dependence in the telescope triggering, image selection, and fluctuations in the observed amount of light.