One of the easiest ways to practice tuning is to actually start with the guitar in tune and then detune one string. When you do this, always take the string down in pitch (i.e., loosen it) as it is easier to tune "up" to a given note rather than "down" to it. As an example, de-tune the 4th string (D). If you strum a chord now, the guitar will sound out of tune, even though only one string has been altered (so remember that if your guitar is out of tune it may only be one string at fault.)
Following the correct method, you must tune the 4th string against the D note at the fifth fret of the 5th string. Play the note loudly, and listen carefully to the sound produced. This will help you retain the correct pitch in your mind when tuning the next string.
Now that you have listened carefully to the note that you want, the D string must be tuned to it. Play the D string, and turn its tuning key at the same time, and you will hear the pitch of the string change (it will become higher as the tuning key tightens the string). It is important to follow this procedure, so that you hear the sound of the string at all times, as it tightens. You should also constantly refer back to the correct sound that is required (i.e., the D note on the fifth fret of the 5th string).