Open the storm door with the lock in the "lock" position.
Close the door and note where the door latch and the lock hit the frame of the storm door.
Hold the plate where you think it should go and make sure the latch fits snugly in place.
Mark the top and bottom of this area with a pencil.
Open the door again to give yourself room to attach the plate.
Drill the beginning of the holes for the strike plate with the appropriate sized drill before trying to screw them in. Many storm doors are thick, and it will be difficult to screw the screws in without pilot holes.
Screw the strike plate between your marks. Have a helper hold the plate in place while you secure it.
Close the door and see whether the plate requires adjusting. If it does, remove one or two of the screws and adjust as necessary.