That's a good question, and one that will get you as many answers as the people you ask.
The safest way to do it is to sell the car to yourself for your true cost. There will be no tax liability on the dealership side because you didn't make a profit. Yes, you will pay sales tax and license fees on the personal side, but that is just part of owning a car.
The easiest way to do it is just drive a vehicle that is in your dealership inventory by putting a dealer plate on the back. Is this the "correct" way to do it? Probably not in CA, but I don't really know.
The way NOT to do it is to sell the vehicle to yourself for 10% of the real value of the vehicle. It doesn't pass the smell test at DMV, and it is about as clear of an example of tax manipulation as exists. Don't sell yourself a $40,000 Lexus for $4,000 with the "leg" you're planning to stand on being anything along the lines of "it is my car and my dealership and if I want to lose $35,000 on a car then that's my business". It doesn't work like that when you are the buyer and the seller.