Another option for the above exercise is to play the two notes on the D string (or the second to highest string for any extended range players). While you play the notes, make sure the two fingers that are not active are anchored to the G string. You'll want to actually pluck each note, rather than doing an official "trill", and you also want to make sure to lift each finger that's active, even the one playing the lowest note, when that finger isn't holding down the note. So as an example using combination #1 from above: The index and middle finger are involved in playing the notes on the D string, frets 7 and 8, while the ring and pinkie fingers are anchored to the G string, frets 9 and 10. You pluck the first note while your index finger is on fret 7 of the D string; you then bring your middle finger down and pluck the note on the 8th fret while simultaneously you lift your index finger as your middle finger comes down. You then bring your index finger back down while simultaneously bringing the middle finger back up and pluck the note again fretted with the index finger on the seventh fret. Remember that while your doing all this, your ring and pinkie finger are still anchored to the 9th and 10th frets of the G string... and they don't come up until the exercise is done. Use the same examples as above but make sure to lift whatever finger isn't fretting on the D string and anchor whatever fingers aren't involve to the corresponding frets on the G string. One note of caution: This exercise really isolates the muscle groups in your hand so you don't need to do each combination for very long. I use it as a quick way to warm up my left hand; maybe 2-3 minutes total for all combinations. You could go a bit longer if you're trying to build up stamina but be careful not to over do it.