get a chromatic tuner, that will help you do it accurately
Quote by FrenchyFungus

Awww, thanks Frenchy

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think of it this way.
to tune to drop d, you'd put your finger on the 7th fret.
you want to go to e flat though.
so you put your finger on the 6th fret.
de-tune that string so it's the same as the A.
then do the rest from the 5th, 5th, 5th, 4th and 5th. just how you would normally.

or you could buy an electric tuner!
Quote by NickKarlsen
i have a tuner.. But can it tune a half step down ?

get a capo. place it in the first fret then tune it to standard. then remove the capo