I was all set to put together a tutorial and realized it just is not that complicated, having done it a few times now, just tedious. However, if you want a tutorial, I will put it together.
I made some changes to lightbox-test
First of all, the size is just a matter of making the frame the right size to fit the video. In other words, I cheated. But I had to do some guesswork as to the size of the design elements. A 1280X720 video does not embed that way. It is taller to account for the YouTube title at the top and the player elements on the bottom.
I played the video on YouTube in Theater mode. The embed code showed 1280X750, but then they add a title. So I changed the 750 to 780 and that seems to work.
1. I created a page and set the page properties to the following:
The I dragged the LightBox Display from the Widgets/Composition palette. I got rid of the stuff I didn't want. like the right and left arrows, and the X. If I really tried, I could probably come up with a theater themed image to put behind the lightbox and just delete all of the border elements. Instead, I just changed the fill to black after adding all of the videos. Having the individual parts made it easier to get the videos all in the same place in the frame. But I had to wait until I was done using them. And I had to set the fade speed to zero or they showed up.
Like I said, try an image instead.
Anyway, you enter the HTML, drag the video into place, and then go back and forth from one thumbnail to another watching to see if the frame is in exactly the same place. Trust me on this, you will want to make the border elements easy to see before you fill them with black at the end.