Start with the sixth string and bring it to concert pitch.With that done.Fret the sixth string on the fifth fret ,this will give you the same note as the 5th string open.(an A note)play both the fretted sixth string and the open 5th string at the same time. bring them as close as you can to sounding the same.If you listen closely you will here a pulse beat.This is the two strings vibrating slightly out of sync with each other.the Pulse is like a waving sound.The closer the two notes are to being in tune with one another the slower the pulse (wave) becomes. The further apart the pulse (wave) becomes the further out of tune you are.The idea is to make the wave or pulse go away.When it goes away the two strings are then vibrating in sync with one another and are in tune with one another.
When a string is fretted on the fifth fret it makes the same note
as the open string below it. The only exception to this is the
fourth string must be fretted on the fourth fret to make the B note
of the open 2nd string.See the diagram in the top left for a visual.
Now i would like to recommend that you go out and buy an electronic
tuner.The reason being is that some people have a difficult time
learning to tune a guitar.So much in fact that it causes many
would be players to give up .You can not make a guitar that is
out of tune sound good no matter how good you are.Plus the electronic
tuner will assure you that you are at 440 concert pitch.If you
find it difficult to tune your guitar then you should consider buying
one they are less then $20.