So, I'll assume you're in the previous city/town if anything and want to progress to the next. Therefore, I'll just reference Anistar City as my starting point. Thus:
1) Start at Anistar City
2) Go to the right of Anistar City and head down, making your way to Route 18
3) At Route 18, first person you're see is the guard wearing the blue hat. From here, continue all the way right until you see a cave (Terminus Cave). You don't need to enter this to find your way to Couriway Town.
4) Simply make your way down the stairs and head all the way town
5) Arrival at Couriway Town
(Note for your question: If Anistar City is not the last city you have reached and it is another city/town earlier, inform me and I will add steps accordingly)
Hope I helped! )