Hey johnsag,

It does have a lot to do with muting the strings (with the remaining parts of your left-hand finger) you're not wanting to ring. The tip of your fretting fingers can go past the A and D and lightly touch the E string to keep it from ringing.

But another part is that you don't just want to strum up and down vertically and hit all the strings ALL the time. You can strum sort of diagonally, meaning TOWARDS the body of the guitar. Imagine your hand is an airplane landing across the strings, and the runway you wanna hit is the A and D strings. Try to come in and land on the A and D strings, but clear the E string as if it's a bridge or building or trees. As you practice that you'll get more precise at only hitting certain strings on what would LOOK like a full strum. You're actually just BARELY missing the strings you don't want.

You can practice this by strumming without hitting ANY strings, and then get that diagonal motion going on by slowly bringing your hand in closer to the strings until you're hitting the ones you want. It'll take practice. Just remember, don't only strum perpendicular to the strings, from E to E. Get a more curved / diagonal approach.

But the muting is always there as protection, so you can be looser with your right hand and avoid extra notes coming out when you hit strings you didn't mean to play.

Thanks for the very good question, and I hope that helps!

-Carl.