1. The very first thing you need to know is that learning to tune your guitar takes time.
Some things on guitar can be learned in minutes, some in days, and others in weeks, but tuning will sometimes take even longer, because you have to train your ears. If results come slowly or don't seem to be making any progress, don't be discouraged, just keep working at it.
2. It will help you to know that the ear is a very skilled instrument for taking in sound. Your ear hears four things in each sound: Pitch, Duration, Volume and Tone Quality.
Pitch is how high or low the sound is. Duration is how long it lasts. Volume is how loud it is. Tone quality is the "character" of the sound. If we were to play the same pitch, at the same volume, for the same length of time on piano, clarinet, flute, violin, guitar, doorbell, or car horn, your ear could tell one instrument (or car horn) from another because of the tone quality. That's because each instrument has a different "character" or "personality" of sound. You can prove you have this ability to tell one sound from another by listening to sounds without looking where they come from.
The point I want to emphasize is that you already hear very well. Learning to tune your guitar is learning a new way of using your hearing.
3.The next thing to know is that when you are tuning your guitar you want to listen only to the pitch of the strings. The quality of the pitch will differ between two strings, and this may at first confuse your ear. You'll mistake the difference in quality as a difference in pitch. For example when playing the first string open and the second string at the fifth fret, you may notice that the first string may sound 'crisper', while the second string will sound a little "darker" in quality. The darker quality of the string at first can be misunderstood to sound lower in pitch. (You may use different words to explain how the strings sound to you, but the idea is that the tone quality of each string will sound different). If you understand that the ear hears a combination of pitch, volume, duration and quality all at the same time, it will help you to filter out the quality from the pitch and overcome the basic problem of tuning.
4. The steps involved in tuning your guitar: The first step is to tune one string to a note from another source. You could use another guitar (one that has already been tuned), a piano, or somebody that know how to tune could guide you along.
A better source is a tuning fork. (It's better because you don't need anybody else around or any other instrument. If, for example you learn to tune to a piano, you're going to have a problem if ever you need to tune and there's no piano handy).
A tuning fork is a U shaped piece of metal with a stem on it. The fork is designed to vibrate at a particular pitch. You can get one that gives you the pitch of the first string on the guitar. (Look for one that has the letter E and the number 329.6 stamped on the stem).
To use the tuning fork you hold it by the stem, tap the U shaped fork against something solid, and place the stem (not the tip of the fork) on either the body, or the bridge of your guitar. (For electric guitars can place it on the pickup). You should hear the note which the vibrating fork produced. The note is the correct "source".
You now adjust the first string to match the pitch of the tuning fork. You do this by finding the correct tuning gear for this string and then turning the gear slowly in one direction or the other. After about half a turn you should hear the string change pitch either up or down. This will tell you which way you have to turn the gear to tighten the string (to raise the pitch) and which way to loosen the string (to lower the pitch).
Now compare the sound of the string with the sound of the tuning fork. If the string is lower than the tuning fork, tighten the string to raise the pitch. If the string is higher than the tuning fork, loosen the string to lower the pitch.
Go slowly. Do not turn the gear rapidly. Turn about a quarter of a turn and then compare the string to the tuning fork again. (You'll have to strike them both again). You'll probably have to repeat this process several times. When the string sounds close to the fork make smaller turns.
When you think the first string is in tune, use the following steps (one to five) to tune the rest of the strings. (Remember, you can only tune as well as your ears hear now. With practice, you can become a better tuner). The following steps repeat the process of matching one pitch with another. The difference is that instead of using a tuning fork you will listen to the string you have just tuned, and try to match the next string to this one.
1. Place the finger behind the fifth fret of the 6th string. This will give you the tone of the 5th sting. (A)
2. Place the finger behind the 5th fret of the 5th string to get the pitch of the 4th string. (D)
3. Place the finger behind the 5th fret of the 4th string to get the pitch of the 3rd string. (G)
4. Place the finger behind the FOURTH FRET of the 3rd string to get the pitch of the 2nd string. (B)
5. Place the finger behind the 5th fret of the 2nd string to get the pitch of the 1st string. (E)