When? Baths should be given sparingly. Some people are tempted to wash their pets biweekly or monthly, however bathing your ferret too frequently will cause the ferret's skin to become dry and their coats damaged. Frequent bathing also causes ferrets to become smellier - their body will need to produce more oils to make up for the ones that you have stripped from their skin. I personally only wash my ferrets when I am introducing a new ferret or when they get so dirty that a simple wipe with a warm, damp cloth will not suffice.
How? Fill a sink or tub with enough water to reach to the ferret's shoulders. Filling the tub too full will cause the ferret to feel as though he is in danger of drowning and will cause him quite a bit of stress unless you ferret loves to swim (which is not common). Water should be warm - like a bath for a baby or slightly warmer, but not hot. Lukewarm water, like you would use to wash a dog, will feel cold to a ferret whose internal temperature is higher than that of a dog. Use a small cup or similar to wet the ferret. Be careful not to get water on the ferret's face on in his ears. Some owners find that their ferret enjoys the bath more if they have special bath toys (such as rubber duckies) or bubbles to play with in the bath (baby safe bubble bath or ferret or baby shampoo can be used for this purpose but be sure to read the warning labels and remember: at least some will be ingested by this small animal). Some ferrets will poop or pee in their bathwater in protest.
Shampoo? Use a gentle shampoo. Most human and dog shampoo is too harsh for a ferret's sensitive skin. There are many ferret-specific shampoos that work well for ferrets and are gentle enough not to harm their skin. If ferret shampoo is difficult to come by in your area, a gentle, hypo allergenic baby shampoo or a gentle kitten shampoo can be used as a substitute. Be sure to rinse the ferret well, and often putting the shampoo bottle in the water before the ferret will warm the shampoo and reduce squirmy-ness on the ferret's part. Pick the ferret up out of the water, pour on the shampoo and lather the generous amount of shampoo into the ferret's body, neck and tail. Rinse the ferret thoroughly with clean, warm water. If you notice any redness on the skin or irritation from the shampoo, discontinue use.
Drying? Take your ferret out of the bath and give them a quick wipe down with a clean towel. I suggest providing your ferret with a nice clean pile of towels for them to dry themselves on, and allowing them to dry off at least mostly in a clean room WITHOUT a ferret litter in it - ferrets like to rub themselves on everything to dry off! If a litter is in the room, you'll find your newly clean ferret caked in litter in no time. If their is dirt or dust on the floor, your ferret WILL find it and coat themselves in that too. Once your ferret is a bit dryer, let them out and watch them go crazy running around the house.
Final Tips: Ferrets take quite a while to dry thoroughly - their undercoat can stay damp for hours after a bath so be sure to wash them on warm days, or be prepared to keep your house warm until your ferret is fully dry. Some ferrets will show you how much they 'appreciate' the bath that you are giving them by soiling their bathwater - be prepared to drain, rinse and refill the bath if that happens. Some ferrets actually LOVE baths. I have never MET one of these ferrets myself, but have seen pictures... I am reasonably sure that the creators use photoshop and special effects to get these images... We ferrets can jump high, so washing them in a bathtub can be difficult without an enclosure. Bathing ferrets together can sometimes help 'ease' an introduction - your dominant ferret will be too busy being bathed and drying himself off to remember to be a jerk to the new ferret; the ferrets will also smell the same after the bath and this may further reduce animosity. Some ferrets will do anything to avoid a bath, including climbing the owner. If you find that your ferret gets extremely scared in the bath, holding them and washing them in the shower with you (and putting them down outside the shower once they are clean) might be something for you to consider.