You have to have had at least 100 visitors to your town (can be the same person, just 100 times).
Next time you go to Porter in the train station he will mention how popular the town is.
That will put it on the PWP list.
There are 3 different styles you can choose (fairy-tale, modern & zen).
You pay for it inside the train station.
You can change the style as many times as you want (including back to the origianal design) but you will have to pay for it each time.
Good Luck.