Look for spot number 12 on the map, which is bell services (number 1 is the express cafe, which is where the SOG buses also are. That is a level below the bell services/lobby area). You would exit the lobby, walk past bell services, and continue on down the sidewalk to the main road. After crossing that main road you follow the sidewalk again as it brings you into the Polynesian grounds. (And actually, as you leave the Polynesian they have a nice little sign pointing you towards Shades of Green!)
Military Disney Tips does a picture-by-picture recap of the walking path here:
https://www.militarydisneytips.com/Shades-of-Green/Polynesian-walk.html
It makes sense once you're actually there though