I have been having some trouble with this and I need some help.
I remember achieving this with relative difficulty as well. Here is a map of Route 118, the route which you would currently be up to:
You must walk along the right side of the route and walk through any gaps between the ledges - I've circled them to make them clearer. Through these gaps, you'll eventually make it to the entrance to Route 119.
Hope I helped. :)