A:
Set up the lure then hold the rod out in front of you, then turn off the brake and the lure fall to the ground. Adjust the brake so your lure just barely falls on its own. When you get it set so that your lure is creeping downward at a painfully slow rate, you should be good to go.
As far as the actual cast goes, your thumb will eventually learn what it is doing. The key is to place your thumb back on the line right as your lure is about to hit the water.
The reason backlash happens is because the reel is spinning faster than your line is being pulled out. So, if your lure hits the water, it is going to virtually stop pulling line, while your reel is still spinning. Keep this in mind and it will help you judge when to use your thumb and how much pressure to put on the line.