This short exercise is designed to train you to pick without anchoring your fingertips on the pickguard, and to encourage forearm rotation when playing single notes. You’ll switch from playing a few notes to strumming a big chord, keeping things loose yet striving for accuracy. I'd start with the metronome set at about 60 beats per minute. You can let your fingernails drag across the pickguard as you play the single notes, but don’t anchor your fingertips on it. To damp unwanted strings while playing singles notes, you can bring the heel of your palm in to touch the strings by the bridge, but try to keep the ball of the thumb off the guitar. Also use the edges of the fret-hand fingers to damp the strings above and below the one you are playing. Keep it relaxed, and listen close to make sure it's clean and smooth-sounding.