You can take the 4 train (of the 4-5-6, the dark green line) directly from Grand Central Station to Fordham Road.
The D and B trains (of the B-D-F-M, the orange line) also stop at Fordham Road. The Fordham Road stop on the B and D trains is slightly east of the Fordham Road stop on the 4 train.
If you want to take the B or D, then:
1) Take the S shuttle train west across 42nd Street, from Grand Central (East 42nd) to Times Square (West 42nd).
2) At the Times Square Station, transfer to the Uptown A or C trains (of the A-C-E, the blue line). Note that the 42nd Street stop on the A-C-E is not actually Times Square, but the Port Authority Bus Terminal. That is because the A-C-Eruns up and down 8th Avenue, while the Times Square Station is at 7th Avenue.
But, there is an underground tunnel that connects the A-C-E to the Times Square Station, so you can transfer without exiting the subway system.
3) Take the Uptown A or C to 59th Street-Columbus Circle.
4) At 59th Street-Columbus Circle, transfer to the Uptown B or D trains to Fordham Road.
See the Related Link below for a complete New York City subway map.
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