Well In case you ahven't figured it out you need to have completed the NR police abd restoring the civilian Powergrid. Then you need to build said powergrid in evry state you want to core and then hit a button there.

You get the power grid tech/building by taking vault city and researching the appropriate focus.

yeah but once you take california you basically have 0 ♥♥♥♥♥♥♥ factories

But you get them back quite easily

Ya, it's hard. So I say once you wiped out Vault City, Reno, and the Indians. You should focus on Coring them first. Then build a crap ton of factories. Once, you go to war with the NCR, Just start building "Powergrids" in areas you control.

Should make it easier once you wipe out the NCR. I can't promise that it will work. Just an idea.

Actually this wont as once you conquer you get the californian resitance which might blow up your powergrid. But it is not that hard, check your garrison law and template, do the foci which give you back consumer law and you should be fine.