Firstly, you need to clean up after any accidents ASAP to remove any smell. A solution of warm water and biological washing powder does the job. Just make sure it is rinsed & dry before the ferrets go back. You have a better chance of success if your cage floor is bare, except blankets for bedding.
If you use any shavings etc you are encouraging your ferrets to toilet on it. Fix your litter tray/trays in place by making holes in the sides and attaching with a clip or twist wire.
Put some wood based cat litter in the trays and also some of their fresh poop, to show them where to go. Also, make sure the litter trays you use are big enough.
Ferrets can be choosy, and it can sometimes take a couple of styles of litter tray before you find one your ferrets are happy with. Large corner trays are available for ferrets, but are really only suitable for up to 2 ferrets.
Ferrets usually need to go to the toilet as soon as they wake up, which means all your ferrets will need to be able to fit in the loo at the same time! If you have a larger group of ferrets, a cat litter tray with deep sides may be better; you may need to cut an access hole in the front if your ferrets aren’t keen to climb in.
Some ferrets will also refuse to use a dirty tray, so you need to make sure you change the litter regularly, but in the early days, always leave some soiled litter in the tray as a reminder.
Most ferrets seem to naturally want to use a tray – you just have to make sure you remove all temptations to ‘go’ elsewhere as explained.
Of course there will always be the odd one who likes to keep us humans on our toes!