Step-by-step: Rain hellfire and destruction on a city
I start by using a broad cloud brush to fill in the entire composition with oranges and whites, making a fire-filled atmosphere. I add two layers and paint the silhouettes of buildings, trying to balance the composition. The mushroom cloud will be in the left half of the image, so I add the heavier skyscraper shapes to the right. With a Polygonal Lasso tool, I select shapes for the low rooftops, keeping in mind the vanishing point and horizon. With a broad brush I paint light values in the selection, establishing the flat surfaces reflecting the explosion. I add detail to the skyscrapers, using a repeating brush to suggest separate floors and balconies. With a variety of Scatter brushes, I paint in tons of flying debris on a separate layer. I then apply a Radial Zoom Blur filter to give the debris energy and motion radiating from the explosion. Using my cloud brush, I darken the sky where the mushroom cloud will be. This will add contrast to the giant fireball.