Welcome to GT...
As Julian says, you have described what you need to do pretty accurately. The easiest way to do it is just like the guy in the video. Keep your hand moving in a constant 'down, up, down, up' motion and keep the tempo constant but miss a couple of strokes to get the rhythm, so:
down
(miss up)
down
up
(miss down)
up
down
up
...and back around again
Missing the strokes as above should mean it all lands in the right place... you'll get it with a little patience... Think of it as 8th notes at 4/4 (8 notes per bar) - you can even play all of them at first to get used to playing in time if it helps. Once you get used to it you can vary which notes you play and which you miss and it might sound even better!
I've moved your thread to a better place... :)
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