open G tuning D-G-D-G-B-D
open A tuning E-A-E-A-C#-E
open D tuning D-A-D-F#-A-D
open E tuning E-B-E-Ab-B-E

to go to open G
start in standard
tune the 1st & 6th (e's) DOWN to D
drop the 5th string (a) down to G
(sometimes people want to go up?)
and break a string...
a good rule when tuning down is to tune PAST
the note..and then go up...it just keeps the string
in tension and helps stay in tune more
good luck
post some audio of some slide guitar when
you get comfortable
-G