Is Frontline flea spray safe for ferrets, and how should I apply it?

Frontline is off-label use for ferrets, but is safe and effective. You best wear some rubber gloves and spray the frontline on the gloves and then rub it on the ferrets, including legs and tail and head (be careful not to get Frontline into their eyes.
If you think you have fleas in your house you also need to follow the following instructions:
Unfortunately fleas only hop on the body of your pet when they want to feed, then they hop off again and stay in your pet's environment. So you need to treat all pets in the household as well as the environment (which contains the fleas, their eggs and larvae).
Flea treatment: 
1) Get some flea medication such as a Flea spray or a spot on from a pet shop or your veterinarian and apply monthly 2) get flea powder, put some in your vacuum cleaner bag and vacuum your house and furniture and also non-carpeted areas frequently to remove flea larvae and eggs which live off house dust., Dispose of the bag in the trash outside. Hot-wash all the pet's bedding,
If you do this diligently for a few months you will get rid of the fleas.