You don't "file" a WARN act notice. You directly give the affected employees notice.
Or much more likely you give them 2-3 months severance which is basically the same as saying "you are laid off in 60 days, don't bother coming back to work". Technically they would be breaking the law but the severance would count against the damages under the WARN act so there's no reason to go to court about it.
§2102. Notice required before plant closings and mass layoffs
(a) Notice to employees, State dislocated worker units, and local governments
An employer shall not order a plant closing or mass layoff until the end of a 60-day period after the employer serves written notice of such an order—
(1) to each representative of the affected employees as of the time of the notice or, if there is no such representative at that time, to each affected employee; and
(2) to the State or entity designated by the State to carry out rapid response activities under section 3174(a)(2)(A) of this title, and the chief elected official of the unit of local government within which such closing or layoff is to occur.
If there is more than one such unit, the unit of local government which the employer shall notify is the unit of local government to which the employer pays the highest taxes for the year preceding the year for which the determination is made.
Or much more likely you give them 2-3 months severance which is basically the same as saying "you are laid off in 60 days, don't bother coming back to work". Technically they would be breaking the law but the severance would count against the damages under the WARN act so there's no reason to go to court about it.
EDIT: You might have to file with an elected official actually, the website is a bit unclear: https://edd.ca.gov/en/Jobs_and_Training/Layoff_Services_WARN