California Code, Elections Code - ELEC § 6916

(1) If the alternate elector is present to vote, by appointing the alternate elector for the vacant position.

(2) If the alternate elector for the vacant position is not present to vote, by appointing an elector chosen by lot from among the alternate electors present to vote who were nominated by the same political party or unaffiliated presidential candidate.

(3) If the number of alternate electors present to vote is insufficient to fill any vacant position pursuant to paragraphs (1) and (2), by appointing any immediately available individual who is qualified to serve as an elector and chosen through nomination by an plurality vote of the remaining electors, including nomination and vote by single elector if only one remains.

(4) If there is a tie between at least two nominees for substitute elector in a vote conducted under paragraph (3), by appointing an elector chosen by lot from among those nominees.

(5) If all elector positions are vacant and cannot be filled pursuant to paragraphs (1) to (4), inclusive, by appointing a single presidential elector, with remaining vacant positions to be filled under paragraph (3) and, if necessary, paragraph (4).