The problem with -- and advantage of -- painting with acrylic is that it dries so very quickly. This is an advantage to anyone who paints for pleasure at home because quick-drying paintings are easy to store. The disadvantage comes during the actual painting process. When painting a relatively unchallenging subject like clouds, the fast drying time will make blending difficult and you may find yourself struggling just to make the clouds look natural.
Tape photographs of the sky in a place where you'll be able to see them as you paint. These images will help you throughout the process, and should inform the way you shape the clouds. Notice that different types of clouds have different shapes. While some are wispy and thin, others are large and puffy. Choose photographs that show clouds of the type you'd like to paint.
Paint the background with an even blue color, using a flat, wide brush. As you approach the area where the sky is closer to the horizon, add a little white to the blue. The sky should grow gradually lighter as it appears to get closer to the ground. Paint quickly because the paint will become tacky as it dries, making blending very difficult. The paint you apply at this stage should be evenly distributed and the gradation from darker blue to lighter blue should be smooth. However, If the first layer of paint is uneven and thin, you will need to paint a second layer on top of the first in order to fill out the color and make it smooth. Repeat as many times as necessary until the background looks evenly painted.
Wait for the sky to dry. Since you're using acrylic paint, this should only take a few minutes.
Switch to a smaller, round paintbrush. Dip it in white paint and paint the puffy shape of the clouds on the sky. Look at the photograph for some idea of what shapes are realistic.
Examine the photograph. If there is any hint of gray in the clouds, dip the tip of your paintbrush into the gray and then paint the gray where you see it in the photograph. Usually this will appear in the middle or on the underside of the clouds. If you see no hints of gray at all, dip the paintbrush into the white paint again and paint a second layer of white on the clouds, in order to reinforce the shape of the clouds and give them a fully three-dimensional appearance. The second layer of the cloud must be painted quickly before the first layer dries, so that the two layers are blended well with each other.
Keep the paints wet during the process by spraying a fine mist of water over the canvas and even the palette.
If you wish to make the edges of the clouds wispy, thin the edges of the clouds with a dry brush before the paint dries on the canvas. Use a rag to wipe off the dry brush as it becomes saturated with paint, in order to prevent the brush from smearing paint around on the canvas.