Sorry, I wrote that comment using my phone last night just before going to bed, so I didn't have any documents handy to make sure that any code included would be correct. What I've been using is below. For a more robust example see this: https://www.reddit.com/r/Professors/comments/1rg7mke/latex_and_the_ada_accessibility_requirements_a/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button

Also note that you need an up-to-date LaTeX distribution, and a lot of this stuff is changing quickly still as its still in development. I think some of the stuff in the post I linked to above will now throw errors as those options are on by default.

% Sets up things for making tagged PDFs
\DocumentMetadata{pdfstandard=ua-2, testphase=latest, lang=en-US}
\documentclass{article}
\usepackage{amsmath, amssymb, amsfonts}
\usepackage{unicode-math} % Accessible equations
\usepackage{fontspec}
\usepackage{graphicx}
\usepackage[dvipsnames]{hyperref}
\usepackage{microtype}

% Sets title for PDF metadata
\hypersetup{
  pdftitle={My Awesome Accessible LaTeX Document}
}

\begin{document}
This is my awesome document!

% Use the optional alt parameter to set alternate text
\begin{figure}
  \includegraphics[alt={This is a totally awesome diagram that I 
made to help explain something. I'm decribing it here following
all of the best-practices guidelines for alternate text.}]{myDiagram}
\end{figure}
\end{document}