Does the Judge just sign an order extending it? On what authority?

Also, if your jurisdiction is extending, for how long are you extending until?

With the exception of a half-dozen courts in the state, everyone's current grand juries should be good through June 30 (matching the court's 6-month term). Some counties release a grand jury after 3 months so as to not wear them out, but they can be recalled at any point during the term to resume business.

CCP 19.07 may or may not allow the extension of a grand jury past June 30. In light of what little case law there is, if you take that route you will be safest if any extended grand jury limits itself to old cases occurring during its original term. I would be very leery to hold over a grand jury and present post-July 1 cases to it absent some relief or guidance from an emergency order of SCOTX or the governor’s office. (Which many come as we near the end of June if societal conditions do not approve.)

Meanwhile, some counties are holding grand juries in-person using masks and observing social distancing and making use of empty courtrooms, county law libraries, or other large, unused rooms in their courthouses. Have the sheriff station a deputy at the door and you should be good to go (if you can convince your grand jurors to do so). SCOTX also just authorized "virtual" grand jury meetings and deliberations via video conferencing, but that route could be fraught with challenges (both technological and legal), so tread with caution there unless you are really eager to make new law.