Use a Sponge
Sponging is one of the easiest decorative finishes to add. It looks light and airy, almost as though clouds painted your walls. Good sponges for applying paint (sea sponges, not dish sponges) are available at most hardware and paint stores.
Pour your top coat paint into a roller tray, then dip a clean sponge into the tray. Don't saturate it, apply just enough to cover the surface. Using a light touch, press the sponge against the wall. Rotate the sponge as you continue to dab it on the wall until your desired area is filled.