Learning or re-learning to play an instrument can be both exciting and frustrating. Many troubles I have found that people face when working toward their musical goal could have been prevented by a patient instructor. It is very important to have a teacher who is working with you rather than one who may feel like an opposing force. The key to sucessful learning in private lessons is instruction and support, not orders and frustration. One very good example of this is when teaching a guitar student how to tune.
Tuning a guitar by ear can be a very difficult process and many do not know what to listen for. Tuning by ear is a very useful skill, it can come in very handy when camping, playing guitar with friends, practicing, or any other time you just need your guitar to be in tune with itself. Unless you have perfect pitch, it is unlikely that perfect tuning can be achieved by ear. First however, let's start with defining "tuning by ear". Tuning by ear (for this lesson) simply means to make the strings of your guitar the correct pitch without the use of a piano or another instrument.
Standard tuning from the lowest string to the highest string is as follows E - A - D - G - B - E.
Starting with your first finger (pointer finger) placed up to but behind the 5th fret (second dot on many guitars) pick the E string and then the A string. This is where we need to know what to listen to. This may sound odd to some, but if you have ever hummed along with any constant noise -- a vacuum cleaner for example, or even a blender -- then you have probably heard and felt "oscillation". Oscillations are a funny noise that happens with the peaks of sound waves do not line up.
If you have not heard an oscillation, try touching your finger to the 5th fret of your E string and picking it, then while that harmonic is ringing, do the same to the 7th fret of the A string. You should get nearly the same tone. These two notes should make an almost pulsing noise as an overtone.
Now do it again, and just for fun try tightening the A string just a little while the notes are still ringing. You should hear the pulsations get faster. The closer you are to having a string "in-tune", the slower the pulsations will be, until they finally sound like one note again.
The goal of tuning is to make that pulsing stop. That is when you know the A string is in tune with the E string.
You may repeat this process with your finger on the 5th fret of the A string, D string, G string *4th fret*, and B string back on the 5th fret.
Once you have gotten through tuning your guitar by ear make sure you continue to practice this skill so that it becomes easier and easier. Learning this skill will help train your ear to both hear the differences between the notes and also to be able to play songs by ear rather than strictly by music or chords.
If you have any questions please feel free to email me. Learning this skill can be difficult. It is a good idea to celebrate your successes rather than be hard on yourself because it is not working.