If you can comfortably do sitting down coaster wheelies now, then you're a good way there... First things first, stop doing them. If it's manuals you want to learn, then its manuals you've got to do (obviously!)
Do as Stevie B has advised, but I would suggest that you cover the rear brake and learn to 'feather' it as required to stop you flipping.
Speed is your friend when learning, go as fast as is comfortable
Once you've got a taste for it you'll find your front wheel is lower than you imagine and you're using brakes less and less. If you have a strong desire to do them truly brakeless, then you'll need to ignore me and not cover the rear brake at all.....