Sand the entire group of vertical panels to size. We stacked them together and clamped them with C-clamps and then put the entire group into the vise. You may want to include a few scraps of wood between the clamps and vertical panels to prevent surface marring. Stacking them all together provides a sufficient sanding surface to guide your sander but you still need to confirm that you are sanding squarely to the workpiece - periodically check this. When repositioning the stack in the vise, always keep at least one c-clamp engaged to maintain the alignment of the stack. Once you have sanded the vertical panels to matching dimensions you can unclamp the stack.