These are basic ferret proofing tips that can apply to any room of the house.
To begin ferret-proofing, you need to lie on your stomach and look around. No, I'm not joking. This will give you a 'ferret's eye view' of the world and permit you to see many hazards that you may otherwise miss. Any space that is an inch big or more is, or can be made ferret accessible by determined fuzzies. Cover any spaces that lead to hazardous areas such as your heating system, you wall, the neighbour's apartment or the great outdoors with sturdy cardboard or duct tape. If you own the building in which you live and don't feel like reapplying tape and cardboard feel free to board the holes up. Don't forget to check for gaps between or under your baseboards and under your cabinets! If you have doors that are difficult to shut or have a tendency to swing open, you may want to consider ferret-proofing the rooms behind the doors because, after all, ferrets are excellent at prying open doors and have an uncanny ability to tell when a door is not securely shut. If the door that won't stay closed leads to the great outdoors, consider keeping it locked when the ferret is out and about to ensure that it does not swing open.
Screens are another big concern when it comes to ferrets. If the screens are loose fitting, they are easily pushed or pulled out of place by a roaming ferret. If they are secure, the screen can be torn by fuzzy ferret laws. When it comes to screens, there are two options: either keep your playing ferret away from them or, if that is not possible, consider having your existing screen replaced with some new-on-the-market 'catproof' screen*
*I have not tested this stuff yet myself, so I am not sure how well it works. If it as good as is claimed, great, but keep an eye on Bandit until you are certain that he cannot rip it.
If you are a green thumb, you'll want to pay special attention to this part... PLANTS ARE FERRET MAGNETS! The damp potting soil is absolutely irresistable to ferrets so unless you want the soil spread throughout your house and all your plants uprooted, consider placing them well out of reach or suspending them from the ceiling. If the plants is a large one which cannot be moved then remove about 2 inches or more of dirt from the top of the plant and cover it with wire and lots of large heavy rocks (though smaller ones can go on top for decoration).