to be perfectly honest if you dont know how this works then you be best not doing this sort of thing.
in short this is a quick explanation, as I understand it, some1 else may no doubt correct me.
if the garage is a long way from the incoming supply then by the time you run the cable to it, along with the exported earth you may well find you have a substantial voltage drop,
it may sometimes seem ok on the meter, but remember with the earth we are looking at a possible fault current.!
by exporting the earth you are effectively extending the size of the equipotential zone, not the best idea.
I have actually seen certain installations where the earth conductor in the garage/shed was actually showing a voltage potential to the earth/ground outside!
theoretically this would mean that anyone standing on the ground outside the shed and touching any part of the shed that was connected to the electrical earth that had been exported would receive an electric shock.
as I mentioned, its not really the sort of thing that can be explained on a forum easily,
IMHO if you need to ask this sort of question then you may well be best advised to obtain the services of someone who knows and understands this type of installation.
regards, Albert.