Take out the blocks, arrange them in a lower Manhattan skyline, then get a couple die cast airplanes - then fling them at the towers as hard as you can.
But if you're a truther, it's a little harder. Set the whole thing up months ahead of time - somehow don't tell ANYONE about it. Then place Black Cat firecrackers inside all of the blocks. Then place them in one of the buildings next to it as well. Be sure that you pay everyone off to keep them quiet, or kill them off ahead of time.
Then, orchestrate a false flag die cast plane hostage crisis - four planes, maybe more. Ditch the planes and the passengers somehow then hit the Pentagon with a missile or a Matchbox truck bomb, again, don't tell anyone...
...do I need to continue this nonsense? Can't we just all agree that our government was asleep at the wheel while some religious fundamentalists who were pissed off at the United States for meddling with world affairs fought back in a spectacular Jerry Bruckheimer inspired terror display? Then the terrorists who expected the paper lion to fold were caught with their pants down as the U.S. fought back by overreacting to the point of tearing up the Constitution and depleting the treasury.