TL;DR: Instructions simplified
Edit: You no longer have to be a Beta Tester in order to use the above code, nor a premium member. Anyone can embed a video onto their profile.
Copy Youtube code.
Use this code for custom boxes. You can change the width and height yourself: (You need to be a premium member to use custom boxes.)
Use this code for journals:
You can substitute "youtube" with "film" or "vimeo" in order to embed dA films or vimeo videos. Simply find the video ID for the video you want that corresponds.
Paste code into Custom Box.
Find video on Youtube.
Look at video's URL in the URL bar above or click on Share.
Find video's I