As outlined in my "Fingerstyle Tremolo" lesson, right hand technique is of paramount importance for playing smooth tremolo, the right hand fingers are labelled:

• thumb = p
• 1st finger = i
• 2nd finger = m
• 3rd finger = a

The finger sequence throughout this passage is p-a-m-i, with p playing all of the "bass" notes. I recommend beginning practise of this exercise at a tempo of '50', then gradually pushing up the speed as you get comfortable with the technique. I play it at a tempo of about 150 in real life in my song "Mezcolanza" which can be heard in full here at WholeNotes mp3 section. As I point out in the "Fingerstyle Tremolo" lesson, this is in fact the Classical method of playing tremolo, the Flamenco method adds one more note thusly: p-i-a-m-i and is more difficult to master. This passage puts the fretting hand in different positions on the fretboard and works best if memorized.