To my friend Colin T. 

The advent of tempered (tuned) instruments has brought a new light to stringed instruments tuning. The world standard:  to tune stringed instruments using fixed tuning instruments (clavichord -piano-, organ) has almost completely taken over the need for highly trained ears that can recognize or reproduce a given pitch -perfect pitch-. 
The orchestra tunes at the beginning of a concert using the first violin pitch as a reference. To depend on each musician perception of tune -relative pitch- can result in disaster.
If the guitarist has a reference note to start, he will be able to find the tuning for the remaining strings. After having gone through the tuning a number of times, the guitarists will notice that his brain has learned, to a certain degree, the sound of each guitar string. What is usually referred to as "perfect pitch" is not as much an innate talent but a learned skill, one that just like most other skills require dedication and perseverance.
The use of a guitar tuner, a small tuning pipe or tuning fork is necessary in order to tune your guitar efficiently.
The key note is the high A which is found on string 1, fret 5. This note can be found by simply lifting your telephone receiver. The pitch you hear is the A we are looking for. It is a world standard and will be the same A in the US or in Botswana.

What you do is keep the receiver of the telephone against your ear -or turn on the loudspeaker- so you can hear the A. You stop the 5th fret on string 1 until your ear tells you that the 2 pitches are equal. Tense string 1 or loosen it until the 2 sounds are equal. I often tune my students' guitars over the telephone. You call me and I hear you play each string and tell you to lower or tighten it depending on how high or low the pitch is on each string. It takes 3-4 minutes to tune a classical guitar and it is a pleasure to be able to help you. 

Once you are 100% sure that string 1 is tuned, you proceed to tune string 2.

The way you tune string 2 is by stopping fret 5 on string 2 (E) and tense or loosen the string until it sounds exactly like string 1 (E).

The way you tune string 3 is by stopping fret 4 on string 3 (B) and tense or loosen the string until it sounds exactly like string 2 (B).

The way you tune string 4 is by stopping fret 5 on string 4 (G) and tense or loosen the string until it sounds exactly like string 3 (G).

The way you tune string 5 is by stopping fret 5 on string 5 (D) and tense or loosen the string until it sounds exactly like string 4 (D).

The way you tune string 6 is by stopping fret 5 on string 6 (A) and tense or loosen the string until it sounds exactly like string 5 (A).

Digital guitar tuners have become so cheap that anyone wishing to learn how to tune a guitar MUST get his hands on one. Tuners work in different ways. They either emit a sound that you have to match for each string or receive the sound of your string and tell you to increase  or lower the pitch according to the sound they hear. The table I prepared below is my help to you which resembles the function of hearing the sound of the given string and matching it by tightening or loosening the given string.

E	B	G	D	A	E

-How to string a classical guitar