The DOD defines a net assessment as "the comparative analysis of military, technological, political, economic, and other factors governing the relative military capability of nations. Its purpose is to identify problems and opportunities that deserve the attention of senior defense officials." It is essentially the analysis or and potential response to a what-if scenario.
The Strategic Studies Institute has published several sources on framing, researching and writing a net assessment:
Some examples of published net assessments that may provide some insight with regard to methodology and format: