After you've created a skill, added a thumbnail and selected a skill experience, you can add at least one card.
To upload a Video, click on Video
Click upload and add the video file from your device. (Upto 4 GB, compress if reqd)
Upload a .vtt file for captions or subtitles if you have them (optional)
Enter the video Title (usually the same as your skill title, unless the skill features multiple videos)
Click Close