One of the cooler things about Samsung's lockscreen video feature is the fact that you can use any video that you have on your phone. It works great with video samples taken with the phone itself, especially portrait ones. Have in mind that you can use any video as long as it isn't longer than 15 seconds.
You can also use Samsung's paid video wallpapers from within the Themes app itself, but we found most of these to be pretty boring in comparison with the super-creative videos that one could find on the internet. Still, the included video wallpapers that Samsung will happily sell you will do the trick if you want to save yourself some hassle.
Fire up the Settings app and head to the Wallpapers and themes menu, where you'd normally change your wallpaper. Alternatively, you can simply hold down on your home screen and select the Wallpapers option.
Hit the Gallery shortcut in the upper left.
Still, by hitting the edit button down below you can trim your 15-second video segment.
Finally, when you're done and happy with your new lockscreen, apply it. You're done!