The rocker panels attach to the plastic clips that are riveted onto the car. The rocker panel has a curved profile with a flat portion. The flat portion goes toward the top. Then push the rocker onto the clips. Lastly, there is a stainless steel screw that holds the panel onto the car, up by the fender area. You're done!
If your car does not have the clips on it, then you'll need to get a copy of the manual that shows the location of the clips or find a another '67 and get the measurements.