There are threads on this forum somewhere, but I'm really bad at unearthing them. I hope someone else finds them for you.

In the meantime, I have repeatedly heard that taking a bike that's a bit too small, lowering the seat as low as it gets, and taking off the pedals, can help. The learner sort of scoots around on the bike, perhaps on a very slight, grassy incline, and learns to balance... Then it's time to get the pedals on...

But others, I hope, will be better to help.