In California, a U turn is considered a left turn that keeps going. As a result, a U turn is legal anywhere that a left turn is legal, except when signs are posted otherwise. So in a left-turn pocket/lane, it is both reasonable and expected that people will make left turns, some of which will continue into a full 180 degree turn. People who do U turns are doing what is allowed, and they have every right to do so. If this seems like a problem, then talk to your transportation department to restrict U turns.

I’m not aware of any aspect of a U turn procedure that would be any different than than a standard 90 degree turn: use turn signals, look for oncoming traffic, look for pedestrians, turn slowly as required by the radius, roll out of the turn with careful acceleration.