Use a roller to paint the wall with sky blue latex paint. Allow the blue paint to dry for 24 hours.
Pour 2 tbsp. white latex paint onto a disposable plastic plate.
Wear gloves to protect your hands from the paint. Wet a sea sponge and squeeze it until no more water drips out.
Dip the sponge lightly in the paint. Press the sponge lightly onto the cardboard and make a few "test" clouds to learn how much paint you should have on your sponge. You want to have more spaces where there is no paint than spaces that are painted. Rotate the sponge and press it to the cardboard again, overlapping the areas that are painted. Build up layers of paint, rotating the sponge each time. Cover some areas with several layers, so that they are opaque and leave some areas lightly covered and transparent. Dip the sponge in paint and blot it on a paper plate, as required.
Press the sponge lightly against the wall to create cloud shapes. Rotate the sponge every time you press it. Overlap the painted areas to make irregular cloud shapes.