Practice, practice, practice...

As I recall from when I was a kid and did such things (35 years ago), it started out just pulling back hard all the way, while accelerating and stepping off the back of the bike as it started to fall backwards... so that initially you don't ride the wheelie, but just pull the bike up. Don't worry about staying up, but over do it, and just concentrate on the bike going straight as an arrow up... not veering at all to either side.

Concentrate on going straight back, and work on precision. This will help you to get the balance aspect down for starting the wheelie, then once you are consistent at pulling straight back, start trying to maintain the wheelie by not pulling up so hard, and trying to find the balance point. Always be prepared to put your feet down until you gain the skills to do it right.

Once you get the side to side aspect down for your "launch" then you need to develop the feel for the balance point, using a combination of your balance and acceleration to keep yourself balanced over the back wheel.

Also, whether related or not, at the same time, I was working on riding with no hands, so this may also help the balance aspect.