I am a member of Pitch Vision Academy and I have actually done training for spin bowling.
There are 2 key important things to concentrate on and that's it!
Action (including run-up, balance while releasing ball etc) and Length.
Forget everything else you have heard, learned etc. etc. Start fresh. Focus on one action and that's it. Make it yours.
As for the length, I practiced by drawing a circle with chalk.
Once you done the above mentioned two, you will automatically have control over the line. And last thing is speed, which will also come to you naturally.
Remember, a smart bowler is far more superior and dangerous than a fast bowler.