How do you get from Grand Central Station to Ground Zero by subway?

You can't; there is no subway service to LaGuardia. The only public transportation to LaGuardia is the M60 bus.

The M60 bus runs through Upper Manhattan only (via 106th and 125th Streets) so you'll have to take the subway. The simplest way to get to the M60 from Grand Central is to take the Uptown 4or 5 express trains (of the 4-5-6, the dark green line), 3 stops, from 42nd Street-Grand Central to 125th Street (at Lexington Avenue).

Then get the M60 bus at its 125th and Lexington stop.

Note that the M60 bus does not operate between 1 and 5 am.

See the Related Link below for more information about the M60 bus.