You know, that's a good question. I grew up in the Bronx, and rode ****man street to what is now the greenway to Riverside Drive countless times, and yet, I can't remember exactly I connected the dots. I do know that coming home, I used to cheat and stay on Riverside Drive until it fed me onto the Northbound parkway, which is very wide in that stretch, and sprint down to the ****man Street exit. This, BTW, is the only way you can get to Ft. Tryon Park and the cloisters from that side. (check the map).

There's been lots of construction since I was there last many years back. I believe that there are now two legal or official options.

1- stay on Riverside Drive until just before it passes under the bridge, and there's a pedestrian cut across the parkway on your left.

2- use 158th Street, which connects to the Greenway, with a bit of a detour. Or pick up the Greenway farther south. (I didn't do that because back then the Greenway was a poorly maintained sidewalk, so getting off it at the first opportunity made more sense)

OTOH - though I can't tell you exactly, I believe you can continue on Riverside a short distance, then work your way to the left at the lights at the bottom of the hill, which is how I used to do it. (it might require a mad dash across the Parkway, but I remember having to do that).

Of course, you can climb 165th street and use city streets to get to Broadway and down to ****man, or enter Ft Tryon Park at the top of the hill, then come back out to Broadway, or go down the hill for the mad dash to the ****man exit off the Parkway.

BTW- I'm sure someone else who's ridden there lately will give you more detailed instructions. For me, it's a matter of habit and something I do but can't explain how.