As soon as you enter you should see a straight path near the moss rock. Keep on biking back and forth 'till you see the grass shaking
EDIT:Some people don't seem to believe me and I want to make my answer clear. Any Pokemon that appears in shaking grass can appear in shaking grass. I know this because I always train with audinos.@MTL your unlucky pansage happened to me before and I also got a unlucky panpour. Just wait.
What I mean by this answer CZ is that most of the forest is squiggly and shaking grass will appear anywhere with grass when you move for a period of time. Usually the routes are squiggly and it's just easier to run/bike along the the path given. You just didn't understand the question.