Clear the area between you and your target.
Keep your palm flat and put the kunai in your hand, with the blade facing inward and the "eye" of the handle pointing toward your target.
Bend your arm at the elbow, raise and throw--keeping your wrist straight. Put enough "spin" on the knife so that it turns midair and the blade points toward your target as it approaches.
Alternatively, you can hold the kunai by the handle. Raise your arm at the elbow so the blade is pointing almost straight behind you. Extend your arm forward and release once the blade is pointed at your target. Don't put any spin on the blade using this method.