You need to be familiar with the requirements in the Business & Professions Code that govern fee agreements and billing:
Business and Professions Code Section 6146 6149.5. Prepare form fee agreements that comply with the statutory and ethical requirements and that are tailored for your practice and the type of matters and clients you expect to work with. For instance, specific requirements apply to contingent fee agreements (Bus. & Prof. Code § 6147.5), agreements for representation in medical malpractice matters (Bus. & Prof. Code § 6146), and hourly fee agreements (Bus. & Prof. Code § 6148). When using any form agreements, make sure that you tailor each one for use in a specific client matter, and periodically review the forms to make sure that they remain compliant, and contain the terms you need in order to best protect yourself and inform your client. In particular, make sure to set forth a clear scope of work in each fee agreement that makes clear what you will and will not do in the course of the representation.
The State Bar of California has sample form fee agreements (most recently revised in July 2015) that you can use as a basis for your own forms. Find the link to these at the State Bar website.