Maybe someone borrowed your favorite DVD and lost it. Or maybe someone has owed you money for a very, very long time. Just make sure that the punishment matches the crime. Often, the punishment has to be a pie in the face.
Throw the pie. Aim right for the center of the face. Add a little twist and push if you want the cream to stick to the person's face when the plate is released.
Set up the throw. It's a good strategy to come at them from the side, so they can't see it coming. You will have to spin 90 degrees for this. A forward approach might make for better aiming, but be prepared for an ensuing hand-to-hand pie battle.
Assemble the pie. Use a paper plate, not a metal or ceramic one. Also, only use whipped cream, or some other creamy thin material. Don't use custard. Crust is optional. Remember that some of the pie will probably wind up in this person's mouth; do not use anything toxic.