From what I have heard from Yes California, it requires a state constitutional amendment and then a referendum put before the populace. Are there any other steps?
An official legal secession of a state from the union is unprecedented and has no formal procedure. So the short answer to your question is that they are making it up as they go along
However, it doesn't really matter because based on previous Supreme Court precedent a state is not allowed to secede from the US. This was brought up a few years ago when Texas was talking about seceding. The relevant case is Texas V. White, which rules that states cannot unilaterally leave the union. From the majority opinion (emphasis mine):
By these, the Union was solemnly declared to "be perpetual."
When, therefore, Texas became one of the United States, she entered into an indissoluble relation. All the obligations of perpetual union, and all the guaranties of republican government in the Union, attached at once to the State.
In other words, no matter what procedure California comes up with, Calexit isn't legally happening unless they find a way to overturn Texas V. White (and its not going to happen).
The other option of course is some sort of secession by force, but that is not technically legal.
From Texas v. White:
There was no place for reconsideration, or revocation, except through revolution, or through consent of the States.
So if the other states consent, California could secede. Unfortunately, there is no definition of what it would mean for the other states to consent. A constitutional amendment could definitely settle this. However, the courts could choose to accept other mechanisms as valid consent. E.g. a treaty ratification, a law, or a referendum (which would have to be enabled by a law).
We don't know what would be a valid consent. We just know that they did not receive it during the Civil War. Texas v. White is often viewed as settling this question, but it really doesn't. All it really says is that unilateral secession is not possible. It doesn't try to define what might make for consensual secession.
The Supreme Court could certainly rule that secession requires a constitutional amendment. But it has yet to go that far. Texas v. White only says that either consent or successful revolution are required. It doesn't describe the conditions of either. That task was left for a future court.
Note that the sentence I quoted appears at the end of the paragraph talking about a perpetual, indissoluble union.
This is a matter respecting the integrity of the Constitution of the United States which, if breached, would predictably trigger catastrophic consequences on a global scale. So there are few options. All are unlikely to succeed.
The options are:
- Amend the U.S. constitution to establish a procedure for one or more states to leave the union.
- Agree to secession by treaty with the United States.
- Fight a war with the United States (and win).
Obviously, none of these is likely to happen. So, most likely, the Calexit movement is nothing more than a political stunt promulgated by a faction within state government for political reasons internal to California state politics.