Clouds come in all types of shapes from flat and wispy to straight rows but most seen in drawings are the fluffy, cumulus-type of clouds. To draw these types of clouds, all you really need to do is know is how to draw semi-circles like the letter “C”. You pretty much string together these semi-circles together to make a cloud like the first cloud to the left:
In the next cloud, if you vary the size of the semi-circle from small to large, you get a better looking cloud. For more complex clouds, you can start layering them as seen on the last cloud example by basically overlapping some of those semi-circles.
If you noticed, the simpler drawing underneath each cloud is a simpler version of the cloud itself and if you’re very observant, you may have noticed that the bottom of the cloud can be drawn as semi-circles, a straight line, or a bit jagged in the last example. Not only that, the bottom can be hazy as seen in the top image below:
All you need to do for a bottom like that is draw several lines at the bottom of the cloud instead of just leaving it at one line. You can even leave a trail if you want. In the next example, you can also have smaller clouds trail the main cloud kind of like a mama duck and her ducklings instead of a fading trail like the upper cloud example.
Not all clouds are drawn all fluffy like I’ve drawn above with very distinctive rounded “edges”. When wind is involved, clouds can lose it’s puffy shape and basically follow the current of the air making them look sharp and knife-like.
This tends to happen when the air shifts such as a character doing magic, something is fired into the sky, or a great vortex is suddenly appearing in the air. All you really have to do is first determine the flow of the air as seen by the arrows, and use that as a guide to draw your affected clouds.
Last but not least, as I mentioned earlier, not all clouds are fluffy as I’ve drawn above. Sometimes you see them in rows, in flat wispy fashion like torn cotton candy, or maybe it’s shaped like an animal or person. For tons of references, check out Google and read up on meteorology; how clouds form, types of clouds, jet stream and so on; in this informative page.