Accessibility in Video
Unlike documents, where the source material is modified, video accessibility is done by adding to the source. The primary types of work done on video is captions and transcripts. There are multiple ways to create captions, but a free and convenient method is to upload the video to YouTube, let YouTube autocaption it, and then revise the captions to make sure they match the audio and have proper grammatical marks. For more information on uploading, see the Uploading page. To learn more about considerations for captioning, see the Captioning page. Creating a transcript is done after captioning, and is simply a matter of uploading the captions file to a site that will remove the time codes from it.
All of this deals with captioning for your own content, but if you want to present someone else's YouTube video, you can't just download and reupload it with captions, because of copyright. Fortunately, you can add captions to the video through the Amara service, and share the link to the video in Amara. For more information, see the Captioning others' content page.