This exercise is from my new book Classical Guitar Technique: Essential Exercises, Scales, and Arpeggios. The 122 page book includes: Practice Routines, Tips, 100 Open String Exercises, 120 Giuliani Arpeggios, Scales, Slur Exercises, Shifts, Finger Independence, Barre, Tremolo, Common Harmonics, and much more.
Practicing open string exercises allows you to focus solely on the right hand and really examine your right hand technique. You might think these exercises are easy but you need to ask yourself: Can I play basic exercises as well as a professional guitarist? Could my legato sound be better? Am I truly relaxed and accurate when I play?
Always alternate the fingers in all the combos:
Two finger combinations: i-m / m-i / m-a / a-m / i-a / a-i
Three Finger Combinations: a-m-i / a-i-m / m-i-a / m-a-i / i-a-m / i-m-a