The ability to sight-read music must be counted as one of the most valuable skills for the classical guitarist. It is a key to becoming familiar with the repertoire, which is already extensive and growing constantly.
For our purposes here "sight-reading" will refer to physically playing music on the guitar, and not necessarily hearing music in your head while looking at a score without guitar in hand (though this is an important skill as well, and an aid to sight-reading in general).
Sight-reading can be compared with reading a book. To sight-read well is to see and play what is written in a music score without having to think very much about the basics of music notation or where to find individual notes on the fretboard, in the way that you would normally read a book without stopping to look up all the words in the dictionary! Both presuppose a working knowledge of the language you are reading, as well as the skills needed to interpret it.
There are some differences between reading music and reading a book, however. To read a book you don't have to do so at a preset tempo, there are no key signatures (though "context" might be a similar concept), nor do you usually have to engage in a physical activity at the same time. You could, though, if you were reading a book telling you to perform a particular action - "After adding vinegar, shake well to blend then put in the refrigerator...."
However, while doing this few of us would be thinking: "Now my right hand fingers are closing around the bottle. My other hand is approaching the lid to open the bottle, making sure that the fingers grasp the lid with more pressure between the thumb and index finger, the middle and ring fingers staying relaxed and in the rest position....". If we did, it would take us 5 minutes to open the bottle (just try tying your shoes this way)!
Similarly, if we thought this way while playing a piece of music:
"In measure one there is an A Major chord which I am playing at the second fret with fingers 1,2, and 3 of my left hand. I'm strumming with the right hand in an up and down motion, taking care to use the "a" finger to accent the open e string.... Hold it! What's my little finger doing sticking out there? Gotta relax it! ....", and on through the piece.
Unfortunately, many people DO try and play the guitar this way, which may be one reason they have problems with sight-reading. The more fluid your technique, and the less you have to consciously think about and control it, the more likely you will sight-read well.
Most music doesn't have such explicit instructions written out for the performer (there are exceptions), because these actions are part of the "repertoire" of skills that the music presupposes. In the same way, a recipe doesn't tell you how to open a bottle of vinegar because you already know how to do that!
To summarize so far: Sight-reading for the guitar relies on skills of technique as a prerequisite, and an understanding and familiarity with the written language and symbols of music. These are the basics, the alphabet and grammar of sight-reading. Fluent reading is only possible when basic skills are automatic, so the guitarist is free to concentrate primarily on the music itself. Some experts would even include the ability to play without looking at the left hand as a prerequisite skill for good sight-reading, and there is considerable merit to this opinion.
Sight-reading relies on physical and mental skills, and both need to be practiced separately.
Physical skills are best improved with technical exercises and drills aimed at this purpose. There are many of these available in various publications today (one good system and set of exercises for developing general technique can be found in the four volume SERIE DIDACTICA PARA GUITARRA by Abel Carlevaro).
Then there's the mental side. Perhaps most problems with sight-reading begin here.
First and foremost: if you don't know the notes on the entire fretboard, COLD, you won't be able to sight-read. This is an absolute. It should be possible to see any note on the staff and instantaneously know where it is on the guitar (you can't play them if you can't find them!).
A good drill is to look at single notes and try to play them on every part of the fretboard they can be found. For example, a "c" might be played on the 2nd string, first fret; 3rd string, 5th fret; 4th string, 10th fret, etc. Practice like this until you can play any note in all its positions without pausing to find it first. When you see a note you want your HAND/FINGER to respond and find it immediately without hesitation.
After this comes recognition of groups of notes: triads, clusters, six-note chords, scales, runs, etc. Again, the goal is to be able to see the notes on the staff and to know right away where to play them on the guitar, without hunting around. A good guitarist will be able to play the same chord or group of notes at several different places on the fretboard, depending on which sounds best for the particular piece of music being played.
Then there are all of the other parts of music notation you should be familiar with, including rhythm, key signature, tempo, dynamics, etc. PRACTICE, PRACTICE, PRACTICE!
Mental training is very important, and some of the best practice for this is reading through music WITHOUT guitar in hand. In other words: air guitar. This will help develop the "instantaneous recognition" habit (as opposed to the "instantaneous coordination" which comes from physical practice).
Take a new or unfamiliar piece of music that fits your present playing abilities. Some of the Segovia transcriptions are good, because they make you look at notes outside of conventional "positions", such as playing the melody on one particular string instead of on several strings in, say, the first position.
Practice visualizing, as clearly as possible, how every part is played and where the hands will go, both on the fretboard and for plucking the strings. FEEL yourself playing everything. DO NOT PASS ANYTHING THAT YOU CANNOT FULLY VISUALIZE! All chords, all runs, etc. must be completely understood. Don't be "fuzzy" about anything. That goes for right hand fingerings, too (reverse this, if you're left handed).
Do this with many different pieces and after a while you'll begin to develop a "mental repertoire"; not of memorized pieces, but of recognizable patterns. You'll start to recognize chords quickly, without having to break them down note by note. You'll be able to see runs and melodies as wholes, rather than one note at a time. You'll get into the good habit of noting key signatures, changes in key signatures, tempo markings, dynamics, accidentals, repeats, rhythm, and all the other information that is part of a musical score. This is all practiced without the guitar.
Once you've done this for a while and your technique is developing, you can start reading through new pieces with guitar in hand to work on your hand-eye coordination. Practice making what you see quickly in your mind happen just as quickly in your hands. Do this with fresh material.
IMPORTANT RULE: always preview any new or difficult piece of music WITHOUT guitar in hand before sitting down to play it on the guitar. This alone will make sight-reading much easier. People who have chronic trouble sight-reading probably rarely or never do this; good, proficient sight-readers usually do.
To make this more enjoyable, use music that you like and will want to eventually play. There's no reason EVER to take the fun out of playing the guitar!
As you become proficient, practice in different keys and with different tunings. Chet Atkins transcriptions are a good source of pieces in unconventional tunings (with my apologies to the classical purists out there!). Also, don't neglect tablature! There are centuries of beautiful music there, and tablature is far easier to read than standard music notation (but not always a desirable substitute).
Another tip: practice sight-reading piano scores to get out of the habit of only reading "guitaristic" scores. This will make reading atonal and other modern music for the guitar, which is full of surprises, much easier.
The value of sight-reading cannot be over-stressed. Without it the guitarist is limited to playing by ear (some of the greatest in the world do this with great results, however, few of us have their talent), or crawling at slow speeds through new scores. Imagine being an actor but not being able to read scripts. How would you learn your parts? How many parts or roles could you study and expose yourself to? So it is for musicians and sight-reading. It is the key to becoming "functionally literate" as a musician.
One final note about sight-reading. With few exceptions, it does not mean sitting down and playing through any piece of music perfectly the first time, any more than an actor reads through and performs a play by Shakespeare perfectly the first time.
Sight-reading doesn't mean not making mistakes, and it doesn't mean never having to slow down and look carefully at difficult passages, or even break them down to a note by note analysis on occasion. Music comes at all levels of difficulty, so you will be able to read something's quickly and easily; other things will take more time.
Sight-reading has an undeserved mystique surrounding it, probably because so many guitarists sight-read poorly and slowly. Yet, these same people read books easily enough, so why not music, too? It should be a natural and easy process. If sight-reading is stressed as a skill from early training, and practiced as such, most any guitarist should be able to become a good sight-reader.
Have fun!- John Sloan