Step 8: The light layer
Now, the desaturated image is not quite ready for being used as a layer mask. Remember the white parts make the image opaque, and black makes it transparent. If you used this as a layer mask, it would use the bright parts of the bright image and the dark parts of the dark image- the exact opposite of what we want. To rectify this, invert the greyscale image (Layer -> Colors -> Invert). If your image needs brightening, do the curves operation as before- mine didn't, so I skipped this step.
Copy the bright layer mask image, create a layer mask on the light layer and paste the bright layer mask image into it as before. This should make the brighter parts of the composite image mroe vibrant- it usually has the effect of making daytime sky more vivid blue, increasing the colour gradient and decreasing wash-out.