Hi, there's a couple ways of throwing them.

You can throw them a bit like a baseball, if you're throwing a shuriken shaped similar to a tire iron, use your bottom 3 fingers to grip the bottom point, your index finger to point along the forward point (In the direction you're throwing) and the thumb should be around the center. You should try to throw it with power, and make sure that the blade stays completely vertical. And angling will cause the accuracy to go down.

You can also throw it like a frisbee. Hold it in a way that feels comfortable in your hand, and in a way that will let it safely fly out when you snap your wrist.

Throw just like a frisbee, use your whole arm and hips/spine to get power behind it, then at the moment you throw, flick the wrist and let go of the shuriken.

Text really sucks. I'll try to get you a website that shows some basic throwing methods, I know I have one in my favorites somewhere..