Practice, practice, practice.
For simple U-turns I don’t think there is a special trick or technique you have to be aware of (unlike switchback technique for MTBs where you have to lift the rear wheel).
Find a grass field where you are allowed to ride on, place your bottle on the ground and start making tight turns around it. Try different speeds. You can also place two or more bottles and try figure eights. Try to keep it nice and smooth.
Be aware that at higher speeds the low friction of grass can be a problem, but at least the fall will be soft ;)
At low speeds you’ll have to keep pedaling or you’ll slow down further and lose balance. Try to avoid hitting the front wheel with your toes. At higher speeds and tight turns where you really have to lean into it: Don’t pedal! Keep the inside pedal up or it can strike the ground.
On roads you’ll want to start your U-turn from the very edge of the road, so you have the full width available. Make sure to look over your shoulder and give hand signals before you turn.