Select a chord with which to practice the cross-string trill, such as a D major chord. Ensure all four strings ring out and that you are using the correct fingers on the right hand to pluck each string. Play the open D string with your thumb. Play the A note (located at the second fret of the G string) with your first (index) finger. Play the D note (located at the third fret of the B string) with your second finger. Play the F# note (located at the second fret of the high E string) with your third finger.
Play each note slowly, and use a specific order of fingers to create a cross-string trill. Hold the open D chord shape with the left hand. Pluck the open D string with your thumb and simultaneously pluck the D note (located on the B string at the third fret) using your second finger.
Pluck the A note (located on the G string at the second fret) using your first finger. Pluck the D note (located on the B string at the third fret) using your third finger. Do not use your second finger for this note. Pluck the A note (located on the G string at the second fret) using your thumb. Ensure you use the correct fingers on the right hand for each note.
Repeat the sequence of notes using the correct fingers. Ensure each note rings out clearly, and play them slowly. Gradually increase the speed at which each finger plucks the notes on the right hand to hear a cascading effect. Notice the different attack with which the same notes are played with different fingers.
Choose a chord in another position and practice a cross-string trill, such as G diminished at the fifth fret. Pluck the G note (located on the D string at the fifth fret) with your thumb and simultaneously pluck the E note (located on the B string at the fifth fret) using your second finger.
Pluck the D flat note (located on the G string at the sixth fret) using your first finger. Pluck the E note (located on the B string at the fifth fret) using your third finger. Pluck the D flat note (located on the G string at the sixth fret) using your thumb. Ensure you use the correct fingers on the right hand.
Practice cross-string trills using different chords, and increase your speed over time. Practice a piece of music associated with the Baroque style (such as pieces by Bach) and incorporate cross-string trills.