One of the hurdles to using ESAS effectively is fitting the spectrum for the FOV; even in the most simple case, we need to fit multiple spectra with multiple models and multiple different response files, with a large number of linked parameters. Even for those with Xspec (sherpa, etc.) experience, there are many ways to make mistakes. This section steps through the essentials of such a fit for a simple case where the “source” emission is smoothly distributed over the FOV. We have found it useful to fit the spectra in terms of emission per square arcminute, as this allows quicker comparison between instruments, missions, and fields.