These days it seems most people use electronic tuners, but it’s always good to know how to tune a guitar without one. One of the easiest ways is what I like to call the cross-hand method. To do this you need to start by getting one string in tune, like your A string. Use a piano, a tuning fork, or another guitar … whatever you have.
Now that you have one string in tune, you can tune the rest of the guitar to itself. So try this: play the note at the fifth fret of the sixth string and match it with your open fifth string. Now here’s where the cross hand thing comes in: use your right hand to turn the tuners of your guitar while the finger on your left hand is still fretting the note. It’s much easier and quicker to get a string in tune if you can hear both strings ringing at the same time.
After you have your sixth string in tune (and of course since your fifth string was in tune to start with) you can start tuning your fourth string. To do this, fret the fifth string at the fifth fret and match it with the open fourth string. You can do the same thing to tune the third string (as in fifth fret fourth string to open third string), but when you get to tuning the second string you’ll need to fret the fourth fret of the third string to equal the open second string. When you tune your first string you go back to the fifth fret of the second string to the open first string. So from your low E string up it’s just fifth open, fifth open, fifth open, fourth open, fifth open.
As an alternative try tuning with harmonics. The harmonics method is more accurate and more fun once you learn how. I recommend owning an A-440 tuning fork. They're small enough that I always carry one in my pocket. Start with tuning your fifth string A by playing the harmonic at the fifth fret. This note is A-440 and it should match your tuning fork.
After this, to tune your sixth string you need to play the harmonic at the fifth fret of the sixth string and match it to the seventh fret of the fifth string. When you get these two to match it’s time to tune the fourth string. Do this by the fifth fret harmonic of the fifth string and matching it with the seventh fret harmonic of the fourth string. Repeat this process to tune the third string: once again, fifth fret, fourth string to seventh fret third string.
Now because the interval between the third and second strings isn't the same as the others, you have to do something different. I recommend playing the seventh fret harmonic on the sixth string and matching it with the open second string. You can do the same thing for the first string by using the seventh fret harmonic of the fifth string to the open first string. (See the finger chart for a diagram.)
Of course, you have to know how to play harmonics to do this. The secret to playing harmonics is that you must very lightly touch the string right on top of the fret and pluck the note close to the bridge.
You can check your tuning with octaves. For instance you can play your open E string and match it with the second fret E on the fourth string. One variation on this method is to play the twelfth fret harmonic of the sixth string and match it with the second fret E on the fourth string. You can do this with all the strings.