Okay, here is the link to enter subtitles/translations.
You enter the lines/sentences one by one, and adjust the time and duration each one is shown. Then you need to submit. You don’t have to do it all at once, I think. Unfinished translations are stored as drafts, an you can even submit unfinished ones so others can do the rest. But as the video isn’t that long, I guess that won’t be necesary.
It doesn’t take that long afterall, maybe an hour or so.