There is nothing in the Constitution about a state leaving the U.S., nor is there anything in the Constitution preventing a state from leaving the U.S.. This leaves it up to Congress to create a definition of the procedure. No Constitutional ammendment necessary. (yes I actually read it on a regular basis)
I was under the impression that leaving the U.S. followed the same procedure as a state wanting to join the U.S. - a presentation of arguments to Congress and then a vote by Congress.
An ammendment to the California Constitution??? Who cares? no really, who cares? We have proposals to ammend the California Constitution on every ballot... and just as many pass as those that fail
You're right, there is no Constitutional instrument for State leaving Union, but there is
for State joining Union (US Constitution. Art. VI. Sec. 3). An amendment would be necessary to create Constitutional instrument for leaving Union -- a highly unlikely endeavor.