nnobakht 0 Light Poster

Hey guys,
i have two grids:
http://tinypic.com/view.php?pic=11w7vrr&s=4

the one on the left is automatically filled with fields. The ones highlighted are optional.
The user then has to enter the name fo their fields that map to the required ones (at least) and the name of any other fields that are mapped to the optionals plus any other.
I now need a way fro the user to tell me which field in the left grid maps to which field in teh right grid.
Ive been thinking about how to do it for a while but i just cant think of anything that would be easy for the user and makes some sense. Any input would be appreciated.
Thanks you