Get some flats on…
ride bike in a straight line, with saddle low as it will go (or indeed find a vey shrt post). Do all this Very slowly.
Making sure your rear brake is shit-hot… throw all your weight back as far as you can whilst pulling hard back on the bars.
The aim IS to get the bike to flip out from under you.
when the bike flips out (back wheel goes under, front wheel comes over the top) jump off the back and land flat on both feet.
Do this repeatedly until you can manage to catch the bike with the back brake BEFORE it flips out.
This way you’ll find where the balance point is, much quicker than working up to it. It’s much further back than you think.
When you are comfortable with this technique, you’ll be able to hold the bike vertical, and stationary for a very short time.
If you then transfer this to a slight downhill slope, and let go of the brake once the bike has tried to escape from under you….. it will want to roll forward, and you can then pull back again, and the technique becomes a game of feathering, and pulling back, whilst the slope give you momentum.
All of this worked forme. But then i am a bit of an animal.