with my new students I follow a simple method. each week we study one string, first four frets and open string, first position. I write random notes and the student have to play them. start with whole notes and half notes. start with natural notes, then add accidentals. everybody can read that.
then, second week add a string and add quarter notes. practice new string apart and then mix the two strings
third week add rests, add a string and so on...
-don't go further quarter notes, you just want to learn the notes.
-read the exercices straight, no start over if you make a mistake, go to the end no matter what.
in a month you will be able to read the notes quite fluently, then begin to add rhythmic complexity. keep things all random, no scales or triads or rhythmic patterns.