Start by talking to Nails Newton outside of the Pick and Lute tavern in Taverley. He asks you to steal a seal from Rolo the Stout, a greedy merchant who monopolizes food.
He suggests making a pie that will make him ill for a bit. Grab some fishing bait and wheat (as marked on the map below), then head to the mill. Climb up the ladder, use the wheat in the hopper and operate the winches. Go downstairs, grab an empty pot off the table, and take the maggoty flour.
Return to Nails Newton and he will ask you to catch a raw crayfish. Go to the fishing spot (shown below) and trade Nicholas Angle. You will be able to receive a free crayfish cage. Catch a crayfish, the go to the flax field and a cut-scene will play showing you burying the crayfish, then sometime later digging it up.
Return to Nails Newton once more and he will ask you to take a potato and stick it down a kebbit hole. Trade Head Farmer Jones, just north of the Flax Field, and you can get a free potato. Head up the hill behind the farmer to the west, slightly onto the mountain, and look for a kebbit hole to use the potato on.
Use either the manky crayfish or stinking potato on the pie crust to combine them all. Enter the tavern and use the uncooked terrible pie on the oven in the back corner near the cook. Return to Nails.
He suggests pickpocketing spices from Foppish Pierre, located just outside the watermill. Use the expensive spices on the terrible pie to make a mouth-watering pie. Head upstairs at the tavern and talk to Rolo the Stout.
Open Them Up!
Climb up the stairs and open the chest. Retrieve the seal and return to Nails. He will hand you a forged letter. Take it to Foppish Pierre. He will be dismayed, but return to Nails once more.