Summary: License plates must be mounted securely so they don't swing, mounted in a visible location, and kept clean so it can be read easily. Rear license plates must be mounted at least 12 inches above the ground, but no more than 60 inches above the ground. There is no requirement on the angle or specific location of mounting. You may not install a cover that obstructs or impairs the readeability of the license plate and the license plate number can not be covered.

5201: License plates shall at all times be securely fastened to the vehicle for which they are issued so as to prevent the plates from swinging, shall be mounted in a position so as to be clearly visible, and shall be maintained in a condition so as to be clearly legible. The rear license plate shall be mounted not less than 12 inches nor more than 60 inches from the ground, and the front license plate shall be mounted not more than 60 inches from the ground, except as follows:

5201(f): No covering may be used on license plates except as follows:
(1) The installation of a cover over a lawfully parked vehicle to protect it from the weather and the elements does not constitute a violation of this subdivision. Any peace officer or other regularly salaried employee of a public agency designated to enforce laws, including local ordinances, relating to the parking of vehicles may temporarily remove so much of the cover as is necessary to inspect any license plate, tab, or indicia of registration on a vehicle.
(2) The installation of a license plate security cover is not a violation of this subdivision if the device does not obstruct or impair the recognition of the license plate information, including, but not limited to, the issuing state, license plate number, and registration tabs, and the cover is limited to the area directly over the top of the registration tabs. No portion of a license plate security cover shall rest over the license plate number.