When I was renovating nursery for my boy, one thing I left as is was ceiling. It's covered in "sky" wallpaper, blue with white clouds on it. Then I added some planes, a seagull, and it became obviously sky-themed room all over. White walls added light, but they were were too plain and dull. So...
Surf some pins to find cloud style you like, and draw something similar:
Transfer it on cardboard and cut out with x-acto knife. Don't make edges too wide.
Don't toss the clouds you cut out! Put a double-sided sticky tape on them. That way you can put it on wall to eyeball more precisely, where your stencils will suit best.
Push stencil against the wall and apply paint, obviously.
I didn't paint lot of clouds, just few here and there - to make bound between ceiling and walls. That's how it looks above the daybed: