Nail clipping is a necessary part of proper grooming for your ferret. It is fairly easy to do with some practice....and a somewhat cooperative ferret. The methods I use will not always work with every ferret. Whatever method you're comfortable with and that works for both you and your ferret is the one you should use. You may even discover a method that isn't mentioned here.
Depending on how quickly your ferret's nails grow, they may need to be clipped every 2-3 weeks or as often as once a week. Probably the best times to clip your ferrets nails is when he/she is tired(after running around and playing) or right after they've woken up from a nap, when they are still somewhat sleepy. At these times, they would be calmer and less likely to squirm and fuss. You will need a pair of cat nail clippers or human(baby) nail clippers, syptic powder, and a lickable treat such as ferretone, linatone, ferretvite, nutrical, or even hairball laxative. I use human nail clippers; I personally find them easier to use. If your ferret is calm enough, you can try just holding him on your lap and clipping nails that way. Not many ferrets will hold still for that long, however. Ferrets nails are translucent, or almost clear, and have a thin red vein running through the middle. This is called the "quick". The "quick", if accidentally cut during nail clipping will bleed and cause the ferret pain. If you accidentally cut the quick, use the syptic powder to stop the bleeding. If you do not have syptic powder, then dragging the nail across a bar of plain white soap will also help stop the bleeding. The worst thing to do when you accidentally cut the quick is to panic. Some people have seen immense amounts of blood and thought the ferret may end up bleeding to death, but they won't. The bleeding will eventually stop. If cutting of the quick does occur, you may want to stop clipping nails and leave it for another day. The ferret will obviously be in pain and be very stressed.
You can hold the ferret in a scruff(or have someone else scruff the ferret) in one hand and clip nails with the other. "Scruffing" is grasping the ferret by the loose skin on the back of their neck. This does not hurt them if done correctly; it's been called an "off" switch on a ferret. Make sure to support the ferret's hind end on your lap or other surface so as not to put any undue stress on the spine. Most ferrets get rather squirmy after only a few minutes; so try offering a little of the lickable treat to distract him. If you have another person to help you, one can scruff the ferret and offer the treat while the other clips the nails. If you're doing it alone, you may have to alternate between clipping and offering the treat. Another method that I've heard of some people using is scruffing the ferret with their teeth and holding them that way while clipping nails. I've never tried this myself, but I would suspect it would be problematic. The "teeth" scruff causing the ferret discomfort, the weight of a 5-6 pound ferret(or even lighter) being too much and causing strain on the human's jaw muscles, not to mention the amount of fur the "scruffer" will end up getting in their mouths, especially during shedding seasons are a few things that come to mind that would make this method less than ideal. So I wouldn't recommend using it.
By far, the easiest way of clipping nails is the "ferretone on the belly" trick. This involves putting a small amount of ferretone(or other lickable treat) on the ferret's stomach and while they are distracted by the tasty treat, you can clip their nails in no time. By the time they are done licking all of the treat off, you're already done with most if not all the nails. First scruff the ferret and lay him acroos your lap on their back. Put a few drops of the treat on their stomach and don't release them from the scruff until they strart licking their stomach. I have a few that immediately jump up and try to run away when they feel wetness on their stomach. I usually end up with the treat all over my shirt while trying to catch a scrambling ferret. Once they are licking the treat, start clipping the nails on the front paws. The front nails grow much faster and longer than the back nails, so always start with those. If you look at the side of the nail, you'll be able to see the quick much easier. Cut the nail as close to the quick as you feel comfortable with. Not too close though. If you cut too close to the quick, it will squeeze the sides of the nail and pinch the quick which won't cut into the quick itself, but it will cause a short burst of pain and may make the ferret jump a bit. If you are unsure of how far back to cut, just cut the sharp tips off until you feel more confident. You don't need to do all four paws in one sitting. Do whatever you are able to and if at any time your ferret becomes overly stressed, then stop, offer a treat and a little praise, and try again at another time. The first few times will be the hardest on both you and your ferret, but after that you should have no problem getting your ferret used to it. Afterwards, you may want to use a nail file to smooth the tips down a bit as they can be a bit sharp once they've been clipped. And always end your nail clipping sessions with an extra treat and lots of praise.
Clipping nails is extremely important for the well being and safety of your ferret. Nails that are too long can get caught in the ferret's bedding. It isn't the actual tips that get caught in the bedding; as the nails get longer the undersides start to split and it's these split areas that get hooked in the bedding. This can be dangerous as they can actually tear the nail from their foot while trying to free themselves. And if they are not able to free themselves and you are not around to help, they can seriously injure themselves(twisting limbs, etc.) and may get trapped in their bedding to the point of not being able to access their food, water, or litterbox. Letting the nails grow too long will also cause the quick to grow too long as well. As ferrets get older, when their nails grow, they tend to curl under and become thicker; you don't want the nails to curl under to the point of injuring their paw pads. And the thicker they get, the harder it is to see the quick. Excessively long nails can also affect your ferret's gait, as well.
Clipping nails is an important part of maintaining your ferret, but it can also be a bonding experience as well.