This 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.
Before playing the following tremolo exercises, consider your overall technique. I often observe students wishing to have blazing fast and even sounding tremolo while their scales and arpeggios have not been fully studied. How can we play tremolo if our i-m, m-a, and a-m-i scales are lacking control? How can we play tremolo if simple arpeggios have not been brought up to speed? As with any technique, a fast and even tremolo is reliant on accuracy, control, relaxation, and a well-rounded technique overall. In all fairness, a student who can play their scales and arpeggios well (with all finger combinations) will be able to play tremolo with ease. That said, tremolo is a specific type of right hand technique so practicing these exercises should help organize your right hand.