Cal. Civ. Proc. Code § 700.120

To levy upon a negotiable document of title, the levying officer shall:

(a) If the negotiable document of title is in the possession of the judgment debtor, take the negotiable document of title into custody.

(b) If the negotiable document of title 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.