Step 5: Add dark layer
Open the dark image in a new window. Copy the contents, then go back to your original image (from now I will refer to this as the composite, and the other photos as the stock). Create a new layer, with Layer -> New Layer, rename it something sensible like "Dark layer"- the initial colour settings are unimportant. Paste the dark stock into it, and hit Anchor. You should now have the middle image on the background layer, and a new layer with the dark stock in it above that- the window holding the composite image will now show just the dark layer.