Spend some time to get the skin stiffener to follow the contour of the skin. This should be done before you draw the outline of the stiffener onto the top skin so you know where to cut. This will pay off in the next step with a very nice fit.
The top skin gets dimpled and the stiffener gets countersunk at the #6 holes. The rivet locations get countersunk through the skin and double countersunk on the underside of the stiffener. The reason is that no rivet tail should stick through the stiffener as the panel sits there.