The best way to teach your ferret to walk on a leash or harness is by putting the harness on him and letting him get used to the sensation of it on his body. Every one of mine rolled over and tried to get out of it. Don't attach the leash until your ferret is comfortable wearing his harness. Put it on him every day for an hour or more, and once he's able to wear it without panicking attach the leash.
This is where the spoon and stick is really needed. Let him drag the leash around for a while, then pick it up and show him the spoon with the treat. As you are holding the lead with one hand, move the spoon in front of him with the other. Don't drag your ferret or force him to go in your direction, lead him with the treat. Practice this with him every day until you can do it without the bribe.
Some ferrets are afraid of the outdoors, there's lots of strange smells and noises out there and it may take a while for your ferret to feel comfortable away from his safe environment. Take a stroll down the block with your ferret in your arms. Take him for short trips in your car. Let him see that nothing bad is going to happen to him if he goes outside with you. Then try taking him for a walk on his leash. Believe me, it will be an adventure for both of you.
There is one thing I should mention that you must do before taking your ferret out of the house. Please make sure your ferret has had all of his distemper and Rabies shots. As far as I know there is no cure for canine distemper in ferrets, very few of them survive if they are exposed to an animal with the disease. If by some chance someone is bitten or scratched while admiring your ferret and it is reported, the only chance your ferret has of escaping euthanasia is if he's up on his rabies shots. You don't want a pleasant outing to end up a nightmare.