By ear you would start from standard tuning. Then play the 6th string 5th fret, then the 5th string open. They sound the same, now play the 6th string 6th fret and tune the 6th string till it sounds like the open A (5th string.) Then you just follow the tuning by ear procedure and your a half step down.

Also, you don't have to tune down if your just learning a song (other than drop d and c), but if your gonna jam along to it, then yea, tune down.