Finger the note you would like to play a tremolo on.
Place the bow on the string, about one-third from the tip of the bow. The tip of the bow is the opposite end from which you hold the bow.
Move the bow back and forth using only the top third of the bow. You can create a measured tremolo with a specific number of strokes per measure, or unmeasured in which you simply move as fast as possible.
Place the appropriate finger on the first note of the tremolo. The appropriate finger will depend on the notes that are used and should follow the normal positions for the violin.
Use another finger to play the second note of the tremolo. You will alternate between the first and second note using your fingers and moving with a measured or unmeasured tremolo.
Pull the bow over the string that produces the alternating notes. Alternate between up-bow and down-bow, while the fingers do the work of creating the tremolo.