Classical guitar technique can be divided into a number of areas. Some of these technical areas overlap a bit while others are quite different. Practicing one area will not necessarily help another. For example practicing legatos (slurs) won't do much to improve your arpeggios.
Most classical guitar music uses numerous technical skills within each piece. If you are great at one skill but poor at another the music will suffer. The audience doesn't want to hear a classical guitar player stumble through the hard part. They want to hear beautiful music.
Here is a break down of classical guitar technique into the main areas to be practiced separately.
Classical guitar technique — A chain is as strong as the weakest link
Linear - The fingers of the left hand are trained to relax into a linear position over a single string. This simple line is the basis of left hand technique. It establishes a home base from which the fingers can judge distances and relationships. Scales, especially chromatic scales, up and down the fretboard are good linear exercises.
Nonlinear - Here the fingers of the left hand are trained to develop the ability to judge nonlinear distances and relationships to each other and the strings. Chromatic octaves played in the first position are excellent for this. Each finger plays in the corresponding fret number i.e. 1st finger in first fret, 2nd finger in second fret etc.
Legatos (slurs) - Practice the down motion and the pull off motion separately because they are two different techniques. Then practice the down and off motion together.
Bars - Full bars and partial bars can easily be incorporated into other technique areas. For example, you can practice arpeggios for the right hand while playing bar chords with the left.
Stretches - The ability to separate the fingers of the left hand. You don't have to have large hands to play classical guitar. You need to develop flexability and independence to separate the fingers. Make up a set of simple stretching exercises. For example put the 2nd finger on the tenth fret of the 4th string and play the twelfth fret 4th string with the 3rd finger. Move down the neck repeating the process. 9 -11, 8 -10, 7-9 etc. Make up other stretches for all fingers and play them slowly. The benefit of a stretch comes from holding it but don't strain your fingers.
Shifting - Position shifts with the left hand can be incorporated into other exercises. Scales and legatos utilizing position shifts are a good way to develop this technique.
Arpeggios (including tremolo) for the right hand
Simultaneous notes - This classical guitar right hand technique is extremely important and it is usually one of the most neglected areas of classical guitar instruction. Classical guitar music is loaded with simultaneous notes using two, three and four fingers to pluck at the same time. A player needs to be able to bring any note out louder than the others. The inner voices are often lost if not played well. When four notes are plucked simultaneously they need to sound as four independent notes played at the same time.
Thumb - The right hand thumb can be incorporated into various exercises such as scales and arpeggios but it is also good to do some thumb exercises. One that I use is played, (without looking), free stroke on the following open stings:
1-2, 1-3, 1-4, 1-5, 1-6, 1-5, 1-4, 1-3, 1-2
then from the 6th string:
6-5, 6-4, 6-3, 6-2, 6-1, 6-2, 6-3, 6-4, 6-5
To play classical guitar well skills such as glissandos, harmonics, rasqueados, tambura, pizzicato, golpes and other effects are needed. These skills don't form the foundation of classical guitar technique and don't need to be practiced on a daily basis. Excellent development of these skills is important and can usually be achieved within the context of the pieces you work on under the guidance of a good classical guitar teacher.
I cannot stress enough the importance of studying with a qualified teacher. A beginner wanting to learn classical guitar needs the best classical guitar instruction available.
Accomplished classical guitarists don't need the best teachers. They already play well. It's the beginning and intermediate players that have the most to gain from the guidance of good classical guitar instruction.
Now that the primary areas of classical guitar technique have been covered here's a simple routine of classical guitar exercises designed to develop a balanced technique.
This practice routine will definitely help you become a better classical guitar player and it can be used in conjunction with any classic guitar lessons you might be taking.
Lets assume you're fortunate enough to have an hour a day.
Linear ............................ 10 min.
Nonlinear ....................... 10 min.
Slurs (legatos) ..................5 min.
Stretches ......................... 5 min.
Simultaneous notes ..........10 min.
Thumb.............................. 5 min.
Bars and shifts............... can be incorporated within the other areas
There are many ebooks available in my blog.
There are other classical guitar methods which contain excellent exercises. Go through the material you have. Copy out exercises and categorize them in a notebook according to the classical guitar technique areas listed above. Continue to add classical guitar exercises to your collection and also make up your own exercises
Now here's the easy part...
Practice every day. With the routine above you can have great classical guitar technique.