One of the easiest ways to practice tuning is to actually start with the guitar in tune and then de-tune 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 slightly detune the 4th string (D). If you play 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).