I use a 12' diameter cast net for everything from peanuts to adult bunker. It takes a lot of practice and the right person to show you. A video is a real good start. If you ever see someone throwing it well, ask them for some pointers.
It's hard to explain, but I'll give it a shot. I hold the rope (one loop around my wrist) in my right hand. Then I hold the plastic eye up high enough so that the lead shots are just on the floor. I grab one loop of net a third way down and then another loop of net 2/3's down.You kind of fold it like a garden hose only it is loops of net. Still holding it all in right hand. Now I spread the net on the dock or deck. I space out about 2/3's of the net weights on my right side and 1/3 of the weights on my left side. I grab the right side on the OUTSIDE of the net about 12" above the weights. Now I'm holding 2/3's of the weight in my right hand. Then I reach UNDER the weights on the left side and grab a handful of net about 12" up on INSIDE of the net. Now you have 1/3 the weight in your left hand. If you can picture it, I have the line, 2 loops of net and 2/3's the weight in my right hand and 1/3 in my left. Now you need to throw the net. What you do is swivel your hips to the right and bring the net almost behind you (like you're throwing a frisbee). As you come back around to throw, arc it a little so that the net goes in the air as you release everything in your right hand. Now is the key; hold the left side just until you feel the weight of net opening and pulling. Let the left hand go with an ever so slight tug back towards you. If you think about, you throw the right side out far in a circular motion and release and tug back a little on the left. With the net going in 2 directions, it opens nice! Of course, if you're left handed, reverse everything. Let it sink. If you are on the peanuts, you'll see them light up the net even before it sinks. Start pulling. If you are on the adult bunker let it sink to the bottom or as deep as you can and the pull it all in.