One way would be to take a gigantic meat axe to this monolithic central socialist government and reduce it to the bloated size of the New York State government....at least.
There are so many things that need to be done to economize and shrink this monster.
Just one would be to revamp this system of our convoluted and confusing system of courts and then to cut all benefits to bureaucrats AND judges. No more life tenure and bloated staffs for elected congressrats.
No foreign aid and no constructing gigantic costly embassies like Iraq, et al, under Bush II.
No useless wars.
No more subsidies for energy sources.
And on and on and on.
Like that's going to happen. You could literally defund entirely every agency other than Defense, Veterans Affairs, and Homeland Security and do away with every dollar of discretionary funding they had and you would still be running a deficit. And you know that's not going to happen. So how is he going to do it?
No useless wars.
Well not wait a second there, you're forgetting one of Trumps early points - invade a country then make it pay for the invasion. That might be a money-maker.