how did you do it?

Way A - This is how I did it, but it was in an older version of course. So I don't know how it exactly translates in your game:

1. Do all the conquer focues with the yakuzas and what not and conquer them as quickly as you can, because they will only get stronger and you're only getting weaker. Ignore the war goal you get at the start of the mod for now.
   
2. Rush the Ranger focus tree before they join NCR, pick the defensive past if they've joined the NCR
   
3. Build your industry (1. Civ then mili), grow your army, research the construction techs to get new man power faster.
   
4. Once your ready attack the NCR with the FEV-Focus
   
5. Take care of the Brotherhood once the NCR is taken care of.
   
6. You may use the war goal now and most probably fight against the mormon faction
   
7. Kaiser's Legion and the northern Brother hood are probably the only ones who could stop you by now. The created a faction in my game and it was a brutal two front war. Since they have low infrastructure especially the Legion.
   
8. Conquer any states which are left.
   
9. ...
   
10. Profit

Way B - Only for the heretics!:

1. Open the console
   
2. Type in
   annex all
3. Hit enter
   
4. ...
   
5. Profit

Addition to Way B:
There is also this command: You can annex the unplayable states with it. It is very buggy and only for show, though. I have only used it once and that was half a year ago. So don't ask me how the command works. Look it up.
Edit: Formatting