A great way to tune your guitar is to use harmonics. It allows you to get the guitar perfectly in tune with itself. I still advise using an electric tuner for important situations or playing in a band.
To start you need to get your 6th (E string, the thickest) in tune. You can tune it to a piano, from another guitar that is in tune, pitch pipes or an electric tuner.
If you have no reference point, your guitar will be in tune with itself (this is all that matters for practice) but not in “A440 tuning” which is what all instruments tune to - important if you are playing with others.
To begin tuning play a harmonic at the 5th fret on the 6th string. Let it ring out while you play a harmonic at the 7th fret on the 5th string. These two notes should sound the same. If they are in tune then you will here a constant high pitch. If they are out of tune you will hear it “warble”. This effect is even more obvious on an electric guitar with distortion so it is worth playing about with a rock sound to get used to hearing the strange effect that the out of tune harmonics give. It actually sounds cool and a lot of rock guys use the whammy bar to make the strings a little out of tune and making a cool "warble".
If it is out of tune the 5th string will need to be adjusted. It will take practice to be able to get the two harmonics ringing out at the same time and to recognise if the are sharp or flat (too high or too low) and then tune it.
Because harmonics ring out even if you take a finger off, you can use your left hand to turn the appropriate tuning peg.
Once the 5th string is in tune it is now time to tune the 4th. Play a 5th fret harmonic on the 5th string and a 7th fret harmonic on the 4th string. Tune the 4th string so there is no “warble”.
The process is repeated again to tune the 3rd string using a 5th fret harmonic on the 4th string and 7th fret harmonic on the 3rd.
Because of the tuning of the guitar, the approach needs to change to tune the 2nd and 1st strings. Play a 7th fret harmonic on the 6th string and it should sound the same as the open (no fingers, no harmonic) 2nd string. Get this in tune and then use a 5th fret harmonic on the 6th string (yes the same one you did earlier) to tune the open 1st string.
It is important to learn to tune well for many reasons. If sounding terrible to any audience or friends is not bad enough, your ear will never develop unless you learn to recognise things being “out of tune”.
This is well worth your time to learn, anyone listening will appreciate it!
BTW - the "warbling effect - technical term for this beating effect is HETERODYNE (cheers Bob)