Its not as hard as you may be thinking.
Take your middle finger and place it on the first note, I think its an a flat on the b string, right? Anways put your 2 finger there and over the rest of the strings take your one finger and place it over them. Dont press down, just hold it over the strings to keep them from vibrating. When you strum all the strings on the b string will vibrate and will be the only one you hear