To use the Universal Subtitles editor for Commons videos, navigate to the file in question, scroll to the Transcode status-table, right click the link Download derivative of the file you want to use as the template and copy the link. This link can then be pasted into the Universal Subtitles editor URL field. After creating the Timed Text, paste it to Commons by clicking the closed caps-icon (cc) in the player of the file in question and then Add subtitles.
The universal subtitles project includes several tutorial videos to help with different sections of the interface. These videos are listed below: