As Tiger has said brace across the door aperture then you can remove the door step. What repair panel have you bought for the door step, is it the complete panel that's goes along the rear quarter panel if so this will help to make your life easier to replace the rear arch close out panel.
The close out panel is spot welded to the rear arch, the top flange of the heelboard and the side of the companion bin.
Also the inner sill stiffener tang is welded on top of the close out.

The inner sill stiffener tang although this is a fabricated stiffener and the tang is shorter.

Once you have done this then you can fit the repair section doorstep followed by the outer sill.