A good rule to keep in mind with the FoL passes is that if the wait is 30 minutes or less, it's best to just wait in the line. For example, The Simpsons Ride will never post less than a 20 minute wait, so using it to bypass a 25 minute wait isn't really worth using the one use.
WWoHP is a good place to start if you're arriving before the second hour the park is open (i.e. the park opens at 9, arriving before 10). Otherwise, I'd put it off until the end. Once Potter gets busy in the morning, it'll stay that way until 4-5 in the afternoon.
The way I would do it is Potter, Lower Lot, Lunch at Springfield, Studio Tour, Simpsons Ride, WaterWorld. Then you can start filling in with the other shows and rides as you see fit (Minion Mayhem, Walking Dead, Special Effects Show, etc.) or you can revisit anything you've already done. Dinner at the Three Broomsticks is highly recommended as is getting an interactive wand to do the magic in Hogsmeade.
If you decide to shop throughout the day, make sure to ask about "Package Pickup". Basically, they'll deliver your purchases to one of the stores in the front of the park and you can pick it up on your way out.