I'm trying to play the song here in this video
I don't really have a problem strumming down, I've been using the fleshy part of my right thumb to strum down, I don't know if it's the correct way to do it but it's been producing the sound I've been looking for so far. There's much less surface area on my thumb to hit the string on the way back up though and I don't have a nail, so I'm not really sure what to do. If someone could explain to me how to position my hand in a way that makes sense it would be much appreciated.