Before you begin, upload your video to YouTube or Vimeo.
To add a video to your page, use blocks from the "Video" category. It can be a single video player, two players next to each other, a combination of video and texts or a video pop-up. You can also use blocks from the "Gallery" category to upload a video.
To make a full-page background video, go to the Content panel and paste your YouTube or Vimeo link to the appropriate field.
Learn more on how to make a cover video: http://help.tilda.ws/cover