Open Structure_render.png, hold down Shift and drag it into your Photoshop document. Both documents share exact pixel dimensions, so holding Shift while dragging automatically makes a new layer. Position this beneath the city and name it 'City Wrecked'. Add a layer mask to this layer as well as 'City'. Using a variety of natural media brushes, knock away areas on both masks to reveal the building’s structure.
Further the effect by using the Burn tool to create inner shadows and scorch marks on both layers. Open Sky.jpg again and use Quick Mask to make a soft-edged selection. Copy and paste it at the top of the layer stack, setting Blending Mode to Screen. Scale, rotate and erase areas to create billowing smoke from the skyscrapers and some foreground mist. Duplicate and transform the layer a few times.
Tidy up the Layers Palette (drag all smoke layers into a new folder, naming it 'Smoke'). Open Saucers_render.png, and Shift-drag it into your image under the smoke. Defringe it by 1px (see Step 06) and name it 'Saucers'. Add a layer mask, generate a selection from Alpha 3 and erase any saucers overlapping the buildings. Now clip a levels adjustment layer and set the midpoint to 0.72.