To tune a guitar one semitone flat, you simply fret each string at the first fret, and then tune the guitar with the string fretted.  You can do it one string at a time, or use a capo.
Say the guitar is already properly tuned.  You fret the bass E string at the first fret.  The tuner hears F, and tells you you need to tune down one semitone.  When you release the string, the note will be E flat instead of E.
I used to tune down all the time to play Stevie Ray Vaughan songs.