First things first, get yourself a pair of small animal nail clippers. I like either of these brands:
Human nail clippers can work in a pinch, but I prefer the better control of small animal clippers.
Next, get a bottle of your ferret's preferred oil (whether salmon oil, Ferretone, coconut oil).
Sit down on the floor, with your legs out, and lay your ferret in your lap. Put a little bit of oil on their belly. If you haven't done this before, you may need to show them where the oil is by gently directing their head. Once they figure it out, they should start licking away.
While they are occupied, start clipping. I know a lot of people freak out about cutting too close, but just keep some nail after the quick and you'll be fine! If you do clip too close, have some cornstarch or styptic powder on hand to stop the bleeding.
If your ferret is getting too wiggly, add some more oil, or leave and try again later. They'll soon learn that nail clipping means they get a tasty treat!
Need a visual? Here's an old video I did on nail clipping!