A little bit more info may help. What kind of reel and more important, what kind of rod. More specifically, what kind of action does your rod have? Is it a Medium/Fast, Medium Heavy/Fast etc. Also, how heavy is the weight that you are trying to throw?
Baitcasters backlash for only one reason and that reason is because the spool is spinning faster than the lure can take out the line. There are many reason why the lure may be traveling slower than the spool is spinning, but that is the reason.
If I were you, I would buy a jig or use some other aerodynamic lure that is at the very upper end of your rods weight rating and I turn all of my brakes on and loosen the spool tension cap just enough so that your chosen lure falls to the ground and when it hit the ground, your spool does not overrun. BTW, Spinnerbaits are not aerodynamic!
Then I would get into a nice open area that is free of trees and bushes or anything else that your lure might catch on.
Then I would practice casting. I would use a two handed overhand cast. Put one hand on the reel and one near the bottom. When you cast, try to release your thumb when the tip of your rod is at the one 1 to 2 o'clock position.
DO NOT use all of your strength and try to cast to China.
A nice smooth cast is fine until you get your casting motion down pat.
If and when you are successful casting like this, loosen your brakes up just a little bit. The brakes are there for a reason. There is no sane reason that a noob should be using half of the brakes when you should be using all of them.
Hope this helps!