I believe there is a screw that holds it against the side of the car. pull them out and look on the body for a place where a screw can go. Also the aluminum sill trim usually has a more pronounced bump to accommodate the kick panel so it sits flat against the rocker. That is the problem with aftermarket parts, they are generic to fit many body styles and years and don't fit without some work. One job at a time and you will get there, keep it up