CA Civ Proc Code Section 700.110

(a)

To levy upon an instrument, the levying officer shall:

(1)

If the instrument is in the possession of the judgment debtor, take the instrument into custody.

(2)

If the instrument is in the possession of a third person, personally serve a copy of the writ of execution and a notice of levy on the third person.

(b)

If the levying officer obtains custody of the instrument, the levying officer shall, if the judgment creditor so instructs, serve a copy of the writ of execution and a notice of levy on the obligor. Service shall be made personally or by mail.