Drop D tuning is when you tune your low E string down a whole step to D, so it's an octave below your D string. To do that, you need a reference pitch.
You can use the octave method, using - in this case - the low E and D strings. Play the low E, pick the D and tune down until it sounds like the same note, but in a different octave.
You can also play the low, open D, then the higher-octave D on the 12th fret of the D string. An even better way is to play the harmonic at the 12th fret - by lightly resting one of your fingers above the 12th fret - and then pick your open D string.
It's always a good idea to scoop up from below, to take up any slack at the nut or in a tuning machine. So, go a bit lower than D, then play a 12th fret harmonic. You'll hear how the pulsating sound - known as the beat frequency - slows down once the two strings are in tune.
Now, you'll have a nice, deep D chord with all six strings, and you can play power chords with one finger.
A good way to double-check your drop-D tuning is to play the 7th-fret harmonic on your bottom string. It will give you an A note, which should match the 12th-fret harmonic on the A string. If it's out of tune, adjust until you're zeroed in.
If you're having any trouble, you can also invest in a guitar tuner - see our guide to the best guitar tuners you can buy for more info.