I made this tiny painting (it is 6″ x 6″ on Masonite, in acrylics) by putting a bit of paint on the gessoed board and using a roller to apply it to the surface. I did several layers to achieve this landscape effect.
I scribbled and scraped into the layers to get some interesting marks that look like some kind of vegetation, and I used markers to scribble between layers of pain as well. I also smudged things a bit with my finger if the hard edges made by the roller looked too harsh to me.
As for the puffy clouds, the roller picked up the small dab of white paint I put on the surface and made spots rather than a full line across the image, because there was of course too little paint to coat the roller. I really like this effect, especially if I keep going over the surface until the paint is all gone.
You could get a similar effect by sticking your finger into some paint and then tapping all around the picture. I have done this sometimes, too.