STEP 1 - Layer a sky. Skies usually occur in bands. Do not be afraid to use green for a twilight or dawn. Green occurs quite often in a sky. You don't see it, because your brain tricks you into seeing blue all of the time. Pay attention the next time the sun sets, or rises. I painted wet on wet for this arctic circle twilight scene.
STEP 2 - Choose a subject. I chose a set of igneous magma mountains out from Iceland. I really loved the pointed pinnacles on the peaks at Skaftafell National Park when I visited in September. The black rock gets lighter toward the bottom where the erosion from the peaks accumulates in small stones. The stones appear lighter than the compressed material at the top. Pay attention to the directions the rocks fall, and water runs. Your brush should go in the same direction. I began with wet on dry, and quickly shifted to wet on wet to get dense areas of black.
Never use unmixed black. I had no Payne's grey, so in order to get a hue like that I mixed blue and magenta into a green-hued black.