That's easy. It will be just like it is now, but afterwards the Feds will pass money onto the UN. You wouldn't have a separate international tax return; all of the domestic machinery would still be in places.
After all, how different would that be from the situation now where your taxes are wasted on a lot of useless pork we don't approve of? (Aside from the fact I would REALLY, REALLY disapprove of an international tax).
The difference is that we, the United States, are a sovereign nation, not a collection agency for an international one world government. Is Congress going to try to sneak it in the budget?