The teacup photo was rotated so I leveled that and started to paint in different sized random cloud shapes on different layers inside it. I then masked each brush layer to merge them a bit better. I added several white glow fx behind the clouds at points where they joined or had natural brightness simply by drawing circles with the Selection Tool, filling with white and applying a Gaussian Blur and Glow effect.
Next, I copied all cloud brush layers and merged to one, moved it below all other layers, darkened and applied a Gaussian blur to simulate cloud shadows on the inner sides of the cup. I drew in some more shadows manually with the Brush, particularly at the bottom of the clouds.
Finally I drew in some Lightening strikes with the Pen Tool around the lighter areas I’d created earlier, chose a suitable size Brush & colour and Stroked the paths with “Simulated Pressure” ticked. I added Blur and Glow and adjusted the Layer Blending mode slightly to make the lightening look a little more realistic.