Here's a video on how to tune half a step down, aka Eb tuning, aka D# tuning!
The tuner notes are as follows; Eb Ab Db Gb Bb Eb.
To do this, you either need a good ear or a chromatic tuner. Learn about andy's TC electronic tuner here. Plenty of other chromatic tuners are available.
You need to be in Eb tuning to play songs by Guns n Roses among many others! More info in the video below.