The key in a good tremolo is evenness and articulation.You need to focus on regularity and each note should have the same volume and duration.
First of all you’ll need a metronome.
Then, practice the first speed burst exercise on all the open string. I start at 60 but you can start slower. Always use a speed you’re comfortable at and can control every thing. Practice on the 6 strings.
[Click on images to enlarge]
When you feel comfortable, practice 2 and 3, increasing the speed burst. Keep the metronome at 60 and progressively go to 120.
Exercise 4 introduces P playing on different strings to create a bass line.
Exercise 5 is the same but without the rest. You will play now a continuous tremolo but still at the half of the speed. A tremolo is usualy written in 32nd notes and, at this point we are still playing 16th notes:
Exercise 6 introduces the 32nd notes, still on open strings:
Exercise 7 is the same but with chord changes: C – G – Am – Am – F – G7 – C
Working on a song like Recuerdos de la Alhambra, start with just the chords and then ad your tremolo.