It is now time to weld the new rocker panel. Make use of the old weld spots as a reference for welding the new panel in place. Just use the old weld spots. There is no need to make new ones.
Hold the new rocker panel in place by securing it along its length with locking pliers or small clamps. Use as many locking pliers or small clamps as necessary. Just make sure that the rocker panel will hold while you weld it in place.