There are two potential 'liberty square bridge" routes.
Once you go across the bridge you can take the immediate right on the path that takes you behind the castle next to Cinderella's drinking fountain. Then go left past Philharmagic and you're at PPF.
Or you could walk into liberty square and go right past Haunted Mansion.
Personally I believe that the quickest, easiest route would be to go left at the hub and onto the bridge towards Liberty Square then immediately right on the path to the back of the castle. This way you avoid the bumping and pushing at the castle.
But PPF's queue doesn't exactly fill up immediately so you don't have to get there ASAP like you have to for A&E or 7 dwarfs or Stitch.