Pencil sketch on paper or line drawing in photoshop
4096 x 2035 (250 res) all three bottom checked
Sky color, quick mask, lighten up the bottom of the sky on the horizon. Lighten the side that has the sun or light source.
Desaturate horizon a little. Sky is a complicated gradient, warming some sides.
block in the clouds. not white. off white, work on shadow tones. use a dark tone of the sky color. chalk brush
keep in mind the light direction when you do the highlights and shadows
block in mountains. use tones of shadows in clouds.
as things get closer they get more contrast(y) and saturated
Use extract on some clouds.
use gradient q mask to lighten up side with sun
put distant haze above the cloud at horizon using the gradient bucket.
clouds, use red channel to matte out.
Cloud matte junk: select, modify selection, expand (by 1 px or so)
Defringe
Dodge tool, right click to select midtones, brights or darks
Hue/Saturation: use colorize to monochromatic
don’t be afraid to distort and warp things.