How do I add audio descriptions to YouTube or Vimeo videos?

The majority of video platforms do not natively support separate audio description files in the same way that they support closed and open captions. In order to add audio descriptions to YouTube or Vimeo, you really only have 3 options:

1. Add your audio description to the primary video as part of the video production process. This means that all users who view the video will hear the audio description. 
2. Create a separate video that includes the audio description. You can then link/direct users to the audio description version for those who would like to use it.
3. Use a 3rd party plugin to add a toggle for audio descriptions. There are a couple options for this but most require a paid subscription.