I'm not sure exactly what your having difficulty with but I'll try and explain best I can. If you have a chromatic tuner then to tune down half step simply tune it to one note lower than standard so E becomes Eb and A becomes wither Ab or B# (depending on your tuner but both Ab and B# are the same) If you want to tune by ear its more difficult but I can suggest a method. If you have a capo put it on at the 1st fret and then tune as if you were tuning it at standard. When the guitar is standard tuned with the capo on remove it and the guitar with then be tuned down half step.

Hope this Helps,