With a standard "auto tuner", just hold the strings on the first fret, then tune down until the tuner says they're in tune (so instead of the first fret on either E string bieng F, it's E). Chromatic tuners will let y tune to whatever note you want without any hassle, and Korg makes several inexpensive tuners with "quadra flat" tuning, which allows you to tune down up to 2 whole steps.