Tune your guitar to "concert pitch." Then fret your low E string on the 4th fret (Ab). Tune your A string to that note, so one half step down. Then tune your guitar to the A string (now the Ab string). The whole thing should be down one half step if you do it correctly.
Hi, I'm Peter
Read my post Or, if you have an electronic tuner that lets you tune to sharps and flats, use that.

Sorry...don't know how to put it much simpler than that. You know how to tune your guitar by tuning the strings to one another, right?
Hi, I'm Peter
yes i know how to tune ma guitar i just dont understand how to tune half sep down
Concert pitch is the correct term for the standard E-A-D-G-B-E tuning. So what you do, is after tuning it to the correct E-A-D-G-B-E tuning, you tune your A string down to an A flat by matching its pitch to the 4th fret on the low E string. Then you retune your guitar to the A string, making the whole thing one half step down. Can't explain it any easier than that.
Hi, I'm Peter