Going over the handlebars is the primal fear of every biker. Therefore: feel your way slowly! Find a surface with good grip, preferably a moderately steep tarmac descent. Start with momentum. The cranks are horizontal, the body position is still centred.
Bend slightly (build up tension) and then stretch forwards dynamically but discreetly (release tension). At the same time, apply the front brake in a controlled manner. Result: The rear lifts off. Only increase the intensity slowly.
Work on the interplay between balance and braking for a longer ride on the front wheel.