Here's a good sky photo to use for practicing painting clouds. It's very similar to the one we made in class; white, light gray, darker gray and blue. For simplicity's sake, let's proceed as if all edges are soft. I recommend reading the description of the step-by-step process a couple of times before launching into your first try. Use good, 100% cotton paper if you have it. It will greatly increase the chances of success. 1/4 sheets (11 x 15") or smaller are good for this exercise.
First, wet the paper on both sides so it will stay wet long enough to mix and apply your colors. Use a big brush to work the water into the fibers. Put a little extra water around the edges of the paper. They tend to dry faster than the middle of the page.
Second, add some blue to the brush and mix it on the palette so the paint is uniform. Surround your cloud shapes with blue. If you want to get fancy, you can use two different blues, one for the upper portion and another for the lower. See the difference? Experiment on a piece of cheap paper to see which colors work best for you.
Third, add a very little bit of orange to the blue that is left on your brush. There is no need to wash off the blue before you add the orange. Stay out of the water bucket, please. You already have all the water you need on the paper, plus whatever is left on the brush you used for wetting.
When you are mixing the blue and orange together, try to balance the mixture so neither color dominates. That should produce a good gray. If you find you're getting a purple, add a tiny bit of yellow.
Fourth, notice where the gray occurs on the clouds. I see it on the lower half of the white shapes. Apply the gray accordingly.
Fifth, add a little more pigment to your brush to make a darker gray, and apply a stroke or two wherever you see it in the photo. This step is often overdone.
The sky is a very forgiving subject. Just about anything you create could actually happen. Given this variability, resist the temptation to fiddle with your clouds. Take what you get. As soon as you paint something in front of the sky, like a steeple, or a telephone pole, the whole thing comes together. Below, here are a couple of sky paintings you can copy for further practice.
Give this exercise a few tries, and bring them all to class. The failures are more informative than the successes. Stay relaxed.