Here are a few sweeping pointers:
First, don’t try to pick each individual string – rather, do “rest strokes”. Let the pick fall or drop into the next string in a controlled flow.
Second, don’t use your shoulder. Many students try to push and pull from the shoulder to sweep. The elbow and wrist should control the flow, not the shoulder. Use a mirror, if needed.
Third, keep strings quiet by palm-muting the first 2 or 3 strings of the arpeggio. This will not only keep them quiet once you move to higher strings, but it will place your hand in the best position to keep the guitar quiet.
Fourth, practice with a metronome. Sweeping is not just a fast blur of notes. The rhythm should be even and precise. Practice with 16 notes and triplets. Listen to John Petrucci sweep – the arpeggios have a perfect rhythm and flow.
Lastly – get a teacher that knows what they are doing to work with you on your technique. Sometimes, there needs to be a lot of positioning and re-positioning until you find your groove.
Questions? Email me at firstname.lastname@example.org