First things first, make a blob. We don't need to get into different cloud shapes right now so draw a completely stereotypical cloud. There are usually more blobs at the top and it's vaguely flat at the bottom. Fill the whole thing in the base colour (which it may surprise you to learn, is not white) and get ready for things to get kinda weird.
It might help to think of the cloud as a clump of mostly opaque bubbles fused together. This'll help inform where highlights and shading are gonna happen. Keep in mind, you don't need to use all of the shapes drawn in the final image. You don't even need to draw them in like I have, though to begin with it can be quite helpful.
You'll sometimes see a checkerboard effect in some spots; that is called "dithering." Dithering is used to create a smoother transition between two colours without having to add another colour between them. Overuse of dithering can lead to things looking blurry or sometimes even messy, but when creating something like clouds a slightly "blurry" look is almost what we're going for.
Dithering is a very good way to simulate the wispy texture of clouds, smoke, and similar things, so I've used it here a decent bit more than I normally do. By adding white shading in places I'd expect the sunlight to hit the cloud (from the top-left in this example) and using a good bit of dithering we can create a pretty convincing cloud.
Now, just shade the "bubbles" like you highlighted them earlier. A slightly darker shade, with a touch more saturation and a bluer hue, creates a more interesting image. The principle is the same as previous guides, but with more dithering. Remove any guide lines you may have added and there you have it. Clouds.
