Plenty of good advice here, for me it's just about feeling the fingers on the left, the thumb on the right of the ball, each pressing into each other.
Can also do something more drastic, like practicing throwing backup- you can't do backup with your fingers on the outside. In short don't turn at all to keep things simple.
Absolutely do it with foulline drills first. When you add swing, muscle memory comes into the picture + the hand does funny things when the ball is dropping fast and trying to release.
Finally, if you've been watching a lot of videos and know the theory, then it's more mental than anything else. Try different ways of reminding yourself. If backup doesn't work try reminding yourself of thumb position, wrist position, palm position, etc etc. Any one of these 'reminders' may end up working for you!
