Always remember that accessible design should be your baseline, not a stretch goal.
Design does not need to be complicated, and you do not need to be a graphic designer to create a good document. This is not a substitute for a design course, but this is a great place to start your thinking about design past a wall of black Times New Roman text on a white page.
Accessible design is often an afterthought. Accessible design is not just good for people with blindness and colorblindness--it can help people with learning disabilities, and some practices (like alternative text) can make your content still consumable on poor web connections and less modern hardware.
Deque has some helpful resources on accessible design, if you want to learn more.
The University of Illinois at Urbana-Champaign offers a free Coursera course on accessible design.
This is not a design course. However, here are some free resources for creating documents in.
Canva: a free (with paid option) design platform with plenty of beautiful, attractive templates that make designing graphical documents easier. Not all the templates are accessible, so you should still evaluate accessibility.
Gitbooks: a free (with paid option) documentation creation tool (which hosts this toolkit). It has basic accessibility built-in in many ways, but is not a flawless accessible option by default.
Some design elements to be mindful of include:
captions are the only option for image alternate text
captions of images lack contrast from the page with some settings
limited options for varying the format of text
hyperlinks do not look distinctive on their own
Google Docs: a free design platform. It is a great alternative for those who need a free, easy-to-use option that still has a bit of power for formatting documents.
The Building Blocks of Visual Design is a free guide by the Interaction Design Foundation that teaches basic elements of design. You can read (or scan) the whole guide in 32 minutes.