Once you’ve got your base video (or videos) in Popcorn Maker, adding elements to it is as simple as grabbing one of the “events” from the right hand side of the editor and dragging it onto either the video stage itself, or the timeline below. Once your event is in the timeline you can change the settings, resize it, move it around and otherwise tweak it to behave the way you’d like. And finally just click the share link and Popcorn Maker will give you either a link (or an embed code) you can paste anywhere on the web.
To get started remixing videos, head on over to the Popcorn Maker site. If you want to see some examples, check out new Popcorn Maker projects on Webmaker.org. For more background on how Popcorn Maker works, check out the Mozilla Hacks blog post.