This is the way I do it. On the end of the light fixture opposite of the light attach your ducting hose. Run the hose outside of your grow area and into your fan so that when the fan is started it draws air from insind the grow room. Connect the carbon filter to the exhaust end of the fan.
Any air that was in the grow area is now being sucked out through the light which will cool the light and any odors will be elimanated when that air is forced through the filter. Simple as pie.