if u dont have a tuner or an effects machine etc. One easy and free way is to use a free guitar tab software (Guitar Pro, Powertab Editor etc). Use the tuner there, turn the 6th string down one notch to Eb or D# (same thing). If u use Powertab, then turn it down one notch from E2 to D#2. Listen to the note and tune away. Hope that helped.