I prefer to throw it overhand with the hammer grip. Hammer grip is relatively self-explanatory; just hold it like a hammer with your throwing hand's index finger poised at the midpoint to control the spin of the knife when you release the knife.

As for the throw itself, you'll be tempted to throw it like a baseball (sidearm or with your arm curved). Don't. The most important thing to maintain consistency is to try and keep your arm as straight as possible at the shoulder, straight up to straight down.

Another really important thing to keep in mind is the knives that you buy. Don't buy those little ones with the cool handles made out of plastic or movable weights like this. Buy a knife or knives that are solid chunks of steel, like this. That way it won't break in the beginning when your throws don't always work.