How do you get the M train at Grand Central?

Take the Manhattan-bound 7 train (the purple line), 1 stop, from 42nd Street-Grand Central to 42nd Street-Bryant Park. Transfer at Bryant Park to the M.

Or, walk 4 blocks west on 42nd Street from Grand Central (at Lexington Avenue) to 6th Avenue, and get on the subway at 42nd Street-Bryant Park.