Define evaluation tasks. This can be performed by automated evaluation tools for functional testing, and end-user focus groups for usability testing.
Define a baseline of web technologies. The goal is to make digital communications accessible to as many people as possible, but by defining a baseline of user agents (browsers, screen readers, media players, Etc), and system platforms (Windows, iOS, Android, mobile devices, Etc), will identify the scope of expectations.
Define testing methodology. The evaluation results will depend upon the tools used, level of expertese, and user involvement.
Define test procedures. Testing digital communications across hardware devices, software applications, levels of user experience, and developer expertese, will require standard Persona scripts, assessing specific web pages, and assessing specific dynamic functions.
Define the delivery process. To meet expectations the accessibility evaluation results must be delivered in a format and in a way it can be understood and acted upon. Consider Executive summaries, management implementation overviews, and developer coding techniques. W3C: Template for Accessibility Evaluation Reports