A brief YouTube tutorial by Lauren Bateman explaining how to incorporate up strums into a basic 4-count strumming pattern. Bateman shows that an up-strum motion is already implicit when returning the hand after each down strum, and advises lightly grazing the strings on the upward motion (not attacking them) to produce clean up strums.
Students always ask me how to get rid of those guitar starters on their up strums in their strumming patterns. And I always tell people, you know, most people are able to do down strums 1, 2, 3, 4. But the secret is, when you're doing those down strums, did you know there's an up strum already built in? Watch my hand.
Every time I do a down strum, I have to bring my hand back up to do the next down strum. So when you're transitioning over to up strums, all you have to do is just, on the way back up, graze over the strings. We're not trying to kill the strings. We're just gracefully scraping across the strings. Down, up.
So you can see there's really no difference on doing your up strums versus if you were just doing four down strums or including them in the strum.