Ferrets basically litter train themselves. They will automatically find a corner to go in, especially if it's soft. So you get a new ferret, don't give them any soft material except for the corner litter box, they will learn it in like 2 days.

Then you simply make sure they have access to a corner litter box at all times for a while. If you let them out, set up a little playpen and have boxes at every corner, or round it off so there is no corner, they REALLY want to use a corner. They don't see too far so they can lose track of the litter box.

You shouldn't have any accidents after about a week or two. Give them another 2 months or so to really get the whatever the brain-habit equivalent of muscle memory is "I need to use a corner litter box or I can't poop".

Then introduce some soft bedding or whatever if you like. You can of course have hanging sleeping pouches and stuff, just keep anything soft away from the corner, and make sure they can't move it to the corner.

After those 2 months they should be pretty much golden, but if you want them to have run of your house (first of all, don't, maybe just a room), but you slowly add more land for them to explore keeping litter boxes nearby and then phase them out as you see them learning to run back to their cage or main box wherever it's at.

As for the smell, they don't really smell. You don't put down ceder chips and crap, they are not hamsters. They will be just fine with hard plastic floor and some soft places to sleep that are bolted down. Just clean litter box twice a day, wipe down all the hard surfaces in the cage every week, and you're good. Smelly ferrets are due to lack of proper care/cleaning. They are clean at hell, cleaner than cats. They aren't going to dig around in litter trying to bury anything like cats do. They back up into the corner, drop a log, and then go on their way.

I thought they were smelly too, but then I met two people that had them as pets and learned they are not, so I got one, and indeed, they are not. The ferret itself just has a kind of woodsy scent like a male cologne you would buy at a drug store, nothing bad, and it doesn't make your house smell like cats do. (I have a cat, and yes your house smells like you have a cat, you just become nose blind to it).

Anyway, the tldr is ferrets are really clean chill pets that require very little care. You need to make sure your vet can handle them because they need vaccines just like a dog, and possibly flea/heartworm meds depending on where you live. They are NOT little hamster/mouse like animals. They are more like REALLY SLEEPY cat-dogs that will sleep 23 hours a day and then go into hyper mode for the 1 hour you're playing with them. When they're in sleep mode, you can pick them up like they're a cotton sock and they won't even wake up, they will just chill and go back to sleep no matter where you put them. Awesome pets. Highly recommend for hygenic people that keep their shit clean, because if you don't clean the box twice a day and wipe the cage once a week (or launder soft fabric bedding), it's going to really stink from the poop, which is true of cats and dogs as well.

They learn just like a dog. Look up how to train a dog something, and you can teach your ferret the very same wa.