one way is
Secure the slats on a piece of wood larger/wider than the headboard would be when assembled and having the same space in between them as needed when assembled. Make a curved template the entire width. This will allow the base of your router to follow the template and cut the shoulder.
Depending if the curve is a constant arc, not accelerating on the ends, you could eliminate having a space between them.
Another idea..... probably better, would be to use the curve of the headboard as the template by partially inserting the slats in the groove and use a guide bushing on the router to duplicate that curve on them. Filler pieces in between the slats would eliminate tearout. maybe someone else has other suggestion?
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
Last edited by woodnthings; 12-12-2011 at 07:04 PM.