This is a video explaining how to equip your video with subtitles using the tool Aegisub.
You'll also need site webvtt.org
Here is the standard filenames for subtitles in various languages.
The .vtt file should be entered in https://dfsweb.web.cern.ch/dfsweb/Services/DFS/DFSBrowser.aspx/Services/MediaArchive/Video/Public/Conferences/YYYY/Event/Event_en.vtt
Here are the lines that must be entered in CDS for the subtitles to show:
NB! The 3 LHCathome tutorials are in dfs "WebLectures" and not "Conferences"
NB! You need to have the script written beforehand!