The litterbox will need to be secured in the cage by something. Binder clips work well, so does bungee cord and string. Whatever you use, you will want something that you can easily remove, but not the ferrets. You will also want a few litterboxes to strategically place around your home in places that your ferret will be. Most ferrets do not like to have to travel far to find a litterbox.
Food When looking for a good ferret food, check the back for the ingredients.
Just because a food is labeled for ferrets does not mean that it is necessarily good for ferrets. On the same note, some kitten foods are just as good for ferrets, if not better. Corn should NOT be in the first three ingredients, the first ingredient should be meat, preferably chicken. Ferrets can not digest fiber, and will not get any nutrients out of the corn, resulting in more eating, more waste, and a less healthy ferret. A ferret on a high protein high fat diet with mostly meat will eat less, have less waste, and generally be healthier with more energy and softer fur.