The first step is to use the gradient tool to make the background colour.
Then open up the brush settings and set the brush to 50 , hardness to 0 and spacing to 30.
Then open the Shape Dynamics menu and set the Size Jitter to 100%, change the control to fade and set at 20 and change the Minimum Diameter to 20%.
Then open the Scattering and set the Scatter to 120%, Count to 10 and Count Jitter to 100%.
Next open Texture and set the Scale to 500%, Brightness to 50, set the Mode to Colour Burn, Depth to 100% and Depth Jitter to 0%.
After than, Open Transfer and set the Jitter Opacity to 100% and change the Control to Fade and set to 2%.
Once those settings are set, then use the paint brush to paint the clouds on the canvas. Once they have been painted, add an inner shadow to make it look like it has some depth.
Once done in Photoshop, add the image as then environment like a Corona Sky or HDRI.
