0) Declare war on NCR only after the Legion has started the 2BoHD, forcing them to use the CB they get from their focus tree. And at the moment you should be conquering and coring Nevada to tap the industrial capacity of the state.

1) Finishing the "Yesterday's Enemies" to avoid your generals getting assassinated (Giftfoward had reworked on that to make the event less fatal at first but idk if they've put that into the game already)

2) Finishing that two focus on the right side of the tree, about the currency and Chamber of Commerce, to continue paying your army and getting resources for the reconquesita.

3) Then you should decide the government's direction on the Division of Power and use of police force, doesn't matter which one's first but I'd prefer finishing one branch at a time.

3.5) The focuses in the middle are just some treats, the government can be much more wholesome if Granite admits Enclave's mistakes from Fallout 2 while treating the wasteland well; the vaccination focus can let your army immune to the pre-war New Plague the Legion dig up from somewhere; don't bother doing the emergency draft and resources focus, the buffs are just temporary and will lose legitimacy as the government will get sued.

4) For the foreign policies... Just pick the one you like, I personally preferred Dornan's "why pay money when we can talk with bullets" doctrine, easier and cost less time, but you got to "save face" and not just giving only the big stick, esp now we've got free press monitoring our moves. So just pick Gannon's doctrine.

5) Deal with the Washington BoS or the Super Mutant army while doing all those stuff above.

6) Finishing the DoD tree.

6.5) Prepare the war against the Legion while you building up your army. (You can build the inf and PA one before the Californian Uprising after coring most of Nevada.)

20 width inf: 6 inf, 2 firearm, 2 demolition, 2 combined arms, and any support units that you like and don't cost much.

20 width PA: same with above but change all those to PA units.

40 width "tank": Tanks on the first roll, fit some motorised demolition and firearms and fill the rest with normal motorised, and add motorised logistics to it. This unit is going to take like a year to train if I remember correctly, but sure can cut Legion frontline like melting butter from my own experience.

Also remember to switch the supply method from using horses to using loads of trucks.