Edit an existing album template, or create a new one using the Theater Add-on. In the designer, under Album Setup:
You may set native video dimensions, but it's honestly not very important; you may keep the defaults if you'd like, and that will probably be fine. If there's any problem, you can always come back!
Set your preferences for the video presentation, then save your template.
With a template ready to use, go to Lightroom.
Under Publish Services, create a new album:
To embed video from Vimeo, get the video ID from the video's web address. For example, the typical Vimeo URL will look like this:
And you would just need to copy the ID –- 47564100 -– from the end, then paste it into the Video ID field.
To embed video from YouTube, get the video ID from the video's web address. For example, the typical YouTube URL will look like this:
And you would just need to copy the ID –- nGsTi05YPms -– from the end, then paste it into the Video ID field.