Practising studies (estudios, in Spanish) provides an excellent way to improve your playing technique. This is because pieces of this kind usually isolate a particular aspect of technique, allowing you to focus upon it. In order to do this, a study will usually simplify some aspect of the composition, so that it becomes easier to learn the piece and repetitively practice the highlighted technique. For instance, a guitar study may maintain a relatively constant right-hand picking pattern, allowing the player to focus upon this technique.
The 20 Brouwer Simple Studies (Etudes Simples, Fr.; Estudios Sencillos, Sp.), are good examples of guitar studies. From the first to the last, they each focus upon an important aspect of classical guitar technique, while also always being satisfyingly musical compositions. And these pieces don't always focus upon the "traditional" areas of technique of earlier guitar studies, such as right-hand patterns and left-hand scales and chords. For example, they also deal with: effective use of dynamics; allowing multiple musical lines to be heard within a musical texture; ornamentation; ligados. And these are just a few aspects of technique that are dealt with; these pieces also allow the player to work upon more musical aspects of their playing, such as rhythmic accuracy.
Study No. 6 is a typical guitar study; it is in fact quite traditional, in the sense that it focuses upon a single technique, in this case arpeggiation of five-note chords using the R.H. thumb and fingers. I'll take you through a good approach to studying this piece; it should serve as a good process for other studies, too.
There are two stages to learning (and using) a study such as this Brouwer: firstly, the bar-to-bar process of learning the piece; secondly, the continued technical practice possible once the piece can be played fluently. Both are useful.
Once this piece has been learnt, it can be used to focus upon R.H. arpeggiation technique, and to gain and maintain fluency with this technique. However, in order to learn the piece, to allow this kind of practice, it may be easier to first take this aspect of the piece away, to make it quicker to learn to play the whole piece fluently. This may sound counter-intuitive, but bear with me...
As the right-hand picking pattern is essentially the same throughout the piece, it may be quickest to learn this piece, by first focussing upon what does change: the chords fingered by the left hand. If one breaks the piece down into a series of chord shapes fingered by the left hand, and initially just practises the changes between these, it can make the process of learning the whole piece far quicker, allowing one to then focus upon the true purpose of the study, right hand technique. In particular, it is useful to "maximise" practice time, by paying particular attention to those chord changes which one finds most difficult, and repeating these correctly in order to learn them fluently. This is what the piece reduces to, if you take out the arpeggiation and just consider the chord changes (in fact, only the filled notes need to be fretted, the others are open strings):
Similarly, once one starts to play the piece with the right-hand arpeggiation, it is a good idea to make "mini-exercises" from any changes that are difficult, in order to fluently change between bars (measures). Again, this "maximises" practice time and effort, by applying most effort to passages of the music which are difficult, rather than always playing from beginning-to-end.
I recently started playing this study again, having not played it for many years. I found it particularly useful to practise bars 12-13 as a "mini-exercise" as this seemed to be the first passage with a difficult left-hand chord change, and so making it difficult to play across the bar line fluently. (These are the sixth and seventh chords in the diagram above.)
Also, isolated practise of bars 22-23 (across the change of time-signature) helped me greatly, as here it is difficult to damp the semiquaver open E bass note at the end of bar 22, so that it doesn't ring under the open A in bar 23:
By following these steps (isolating the L.H. changes; extracting small difficult sections to practise alone; then playing through the whole piece), I was able to play the piece fluently in a relatively short amount of practice time. This then allowed me to focus upon the actual purpose of the piece: developing my right-hand arpeggiation.
And, of course, this is just the starting point; once one is able to play the whole piece easily, one can start to use the study to work upon other aspects of technique. Brouwer himself suggests an alternative picking pattern (see below), at the start of the piece, but you could also experiment with: using a variety of different dynamics and tone throughout the piece; accenting different notes within chords in order to create a sense of line in different musical "voices" (holding different notes for different lengths, and choosing whether to let them ring between bars also helps with this).
Although these ideas and this approach deal specifically with this single Brouwer study, I think this approach can work well for most studies (although the specific way to "reduce" a piece will be different for different studies). In fact, I would say that these kind of pedagogical compositions are best studied in this way. Merely playing through such pieces will certainly improve technique, but extracting passages for further study, or devising small exercises based upon short passages, tailored to ones own technical needs and interests, will prove more fruitful. This is then real study and not just playing...