Hello and welcome,
It is not up to the parties to determine whether a worker is an independent contractor or employee, and thus subject to income tax withholding and other requirements. Under CA law, the worker is presumed to be an employee unless the employer can prove otherwise and an improper classification subjects the employer to steep penalties.
Here is a link that provides information in determining how to classify workers:
If the worker is an independent cntractor, then all that is normally needed is a written agreement indicating the terms of their employment.
Here is a link that provides a sample contract:
I hope you found my answer helpful, even if the law is not in your favor; please click on the GREEN ACCEPT button above for my answer. This is necessary for me to be paid for my work and so that I can get credit for assisting you. Your question will not close, and you will still have the opportunity to follow-up if needed. Leaving a bonus and positive feedback is not required, but doing so is certainly appreciated!
If you have additional questions, please keep in mind that I do not know what you already know or don't know, or with what you need help, unless you tell me. Please consider that I am answering the question or question that is posed in your posting based upon my reading of your post and sometimes misunderstandings can occur. If I did not answer the question you thought you were asking, please respond with the specific question you wanted answered.
Also remember, sometimes the law does not support what we want it to support, but that is not the fault of the person answering the question, so please be courteous. Thank you.