How to Muck One Card
One of the things I failed to mention in the video is that when you realease the card you are palming to sweep up the card on the table, you only release with your thumb, you keep the other corner still pinched between your pinky and ring finger. That way, when you sweep up the card on the table, it will automatically be caught between those two fingers and all you have to do is pinch the other corner between your thumb and index finger.
To preform this sleight, you also need to learn how to palm a card. This lesson is available in the "Palming Cards" section.