As I remember the description of the process requires approval of the population of the state and approval of the Congress of the U.S. See Article IV Section 3 of the U.S. Constitution.
I think the movement is the brainchild of these guys, but I don't know for sure. No proposed map yet, but most folks think California would be made up of the coastal counties from L.A. to Marin and possibly two counties east of Marin which would incorporate Sacramento and retain it as the capitol.
Whatever, the new state boundary could be assembled from the same materials as Trumps new border wall and ports of entry established at the north and southern ends.
Anyway the petition to get the White House to respond one way or the other is here.
So pop over and sign it. At worst, nothing will happen. At best, the state of Jefferson would likely elect 2 new Republican senators.