There are no 'rules' concerning this. However, the sporran straps are traditionally worn loose - not passed through any loops at all - and should sit comfortably along the lower edge of the kilt belt, not running on top of the belt.

As for where the sporran itself should be positioned: again, there are no rules but, traditionally, the top of your sporran will be positioned about 4 fingers width below the bottom of your belt buckle. A sporran sitting there will not only look comfortable, but will BE comfortable. Any lower or higher than that is not a good idea from the point of view of your own comfort.