Only with the help of your own hearing
As I wrote above - being able to tune the guitar alone, with the help of your own hearing, is ideal. But keep in mind that it's not just about tuning the strings to match each other, but also to hit the exact frequency of the tone, without which we can't afford to play with other instruments (especially those with fixed tuning). If the guitar is subdued or overcrowded, it does not have to do well with strings, and in addition, when exaggerated overcoat threatens to tear bridge and twist their throats.
Using another instrument's tone
It is also an effective way, but the helping instrument may not always be well tuned too.
Tuning the guitar according to one tuned string
To tune the guitar we just need to have a little hearing and properly tuned at least one string (the most practical is about e1 or e6.) To do this we use a metal or blow tuner or another instrument.
There are many ways to proceed. The most common are:
1) Using the 5th Field Rule
There is logic:
Example: We have a tuned e1 (lower) string.
We will debug the string B - this strings pressed on the 5th array must sound the same as the empty e1 (as there is the same tone). We fine her until it sounds the same. For a better understanding, show the tones on the guitar fretboard.
We attach string G - here is the only exception - the tone h is on the 4th field, so we press the string on the 4th field and proceed the same as in the previous case.
Dash the strings D, A, E6 - press again on the fifth field and fine-tune to the previously tuned string.
2) Using flags