The case you have will determine where the PSU should be mounted. Cases with bottom-mounted PSUs normally have holes or a grill on the bottom so the PSU can draw in cool air from beneath the case. If it does not, the PSU must be mounted "upside-down" (which is not a problem) with the fan UP. If the PSU is able to draw air from beneath the case, make sure not to place it directly on carpet, which may muffle those holes; put it on a piece of wood or fiberboard.
It is especially important for the Corsair "GS" (and "CX") PSUs to get good airflow, as their Samxon capacitors degrade quickly when heated.