Let the ferret choose the location of the box. Watch your ferret to see which corner of the cage it prefers to use as a restroom, and place the box in that corner.
Choose a litter box with sides at least four inches high to prevent the ferret from dropping its "deposits" over the edge of the box.
Use a pelleted litter; clay litters and clumping cat litters are not recommended.
Attach the box firmly to the cage to prevent the ferret from dumping the contents.
Keep an open mind and exercise plenty of patience.