While it's geared toward online content, the Web Content Accessibility Guidelines (WCAG 2.1) provide lots of useful recommendations (and related principles) for all types of media: https://www.w3.org/TR/WCAG21/

Rocky Mountain ADA Center has a great article that reviews the most important information in the WCAG: https://rockymountainada.org/news/blog/twelve-days-web-content-accessibility-guidelines-wcag