All it means in this case is to tune the whole guitar down by a 1/2 step. Then tune the E string down another half step. You probably won't even need a tuner..just do it by ear.
Otherwise, most tuners these days have the ability to drop tune. (at least Korg tuners do anyways).

btw: 'Drop D' tuning is usually meant as just tuning the bottom E string down to a D and leaving the rest of the guitar tuned normally.
It's usually used so you can get a nice bass note on open D chords.