How to use the YouTube web editor
To begin, you need to go to http://www.youtube.com/editor. You should see a screen like this:
You should then see a screen like this:
You can then mix videos to your pleasure.
To see what all the other buttons do, move your arrow on to the button in question and keep it there. A label should materialise telling you what that button does.
For any more information, refer to this post by Michelle.