Some people have only one cowlick at the back, so it looks as though the hair sort of swirls over all to one side at the bottom of the hairline. Others have 2 cowlicks, making the hair on both sides swirl toward the center, looking like a duck's tail.
When cutting a ducktail neckline, you don't really need to do anything out of the ordinary. Just cut as though it wasn't there when dealing with fine or medium hair. However, if the hair is very coarse and straight, it may prove to be a problem. You will need to cut it very short and taper out to a long length further up the back of the head. You can do the taper either scissor-over-comb or clipper-over-comb to get the ducktail area really nice and short.
Font Hairline Cowlick
The best way to deal with a cowlick at the front hairline is to let the hair lie where it wants to. If you don't want your hair to lie in the direction that it wants to, you are going to need extra length. You can do this in 2 ways:
Cut the entire top section 1 inch longer than you usually would.
Leave the first 2 inches of hair (behind the front hairline) longer. Blend it in with the rest of the top when you've finished cutting.
The easiest way, of course, is to go with the natural hairgrain of the hair rather than try to fight it. The famous model, Claudia Schiffer, has not one, but 2 cowlicks at her front hairline. She deals with it by wearing her hair longer and having it styled to compliment, rather than trying to hide the little erratic hairgrain areas. So ..... go with the flow, don't fight it!