Folk musical forms from Russia to Appalachia use the mandolin. Even baroque and classical music composers wrote for the mandolin. This diverse group of musical styles shares one mandolin technique in common: the tremolo or trill. The trill consists of a percussive, rapid-fire series of pick strikes that accent single notes for a singing tone not unlike vocal vibrato. Mandolin tremolo can prove a bit strenuous for the beginner, but you can eventually master the effect by practicing some basic techniques.
Hold a mandolin in a stationary position with a strap or resting on your leg. Keeping the body of the instrument steady allows your picking hand to strum more accurately.
Rest the butt of your picking hand against the body of the mandolin. This can help keep the hand from moving too much while picking. You may also experiment with keeping your picking hand lifted off the body of the mandolin.
Fret a single note on the fretboard.
Hold the pick at an angle to the strings. Experiment with different pick angles as you practice.
Pluck the fretted strings with a downward stroke, then again in an upward stroke. Allow each note to ring out for an even amount of time. Start slowly and gradually build up speed. Play as slowly as you need to fret and pluck each note correctly.
Try fretting other notes as you practice the tremolo technique. Once you have mastered a single-note tremolo, experiment with adding the tremolo to songs and solos.
You can judge what technique suits you best. Practice in a relaxed, focused manner and pay attention to what techniques sound and feel the best and produce the best results.