How to add a name to my deed in the state of California

A new deed is written, and filed. No real difference than a sale, except that a name is merely added. The wording will need to be done, by how they are to be added.
Best to have an attorney or title company do this.

Why do they need to be added?

A lawyer isn't needed, but if you don't use one, know what you are doing!
Once you add someone, you can't change your mind, and unmarried couples who break up often have one person full of regret, while the other one laughs all the way to the bank.
You can ask the deed clerk about the options for types of joint ownership and survival if one dies.
Read this for general types.
Joint Property and Concurrent Ownership | Nolo.com (http://www.nolo.com/legal-encyclopedia/joint-property-concurrent-ownership-32229.html)

Go to the county Recorder of deeds and ask the process. It will vary county to county. Generally, as noted, you just need to execute a quit claim deed transferring your ownership to you and the other party. If it's a spouse it is generally easy. But if not there many be a transfer fee or tax involved.

If it is NOT a spouse, be very careful. Remember you are giving a piece of the property to this person. They will now own a share of your home! There also may be other tax consequences.

As was mentioned... plus.. if the person you are adding to the deed hits a financial hardship... or lawsuit... or by choice... they could force you to buy back their portion of the property you just gave them for free or have a partition suit filed forcing the sale of the property. Or a creditor place a lean against the property. Many bad things can come of this. Consult an attorney to explain all this because once its done... you can't go back cheaply or easily.