There are a few ways you could draw clouds, it just depend on what style you're going for. If you want a very realistic look, work on shading to give the clouds depth. If you want very simple cartoony clouds, you could draw curvy m shapes with a tail that fades away, as if the wind was pushing it a little.
I suppose the biggest thing to keep in mind while drawing more realistically, is that clouds are always changing shape, so don't draw any really harsh blacklines with them. An organic edge, with an outline of off white or grey, blended to fade into the white of the rest of the cloud would probably work quite nicely.
The eraser is also a good tool to use, to make clouds. sketch the general shape you want (you don't even have to do that, really), and after shading/coloring the background, just take the eraser, and have fun with it, in cloud-like shapes.
These are just a few ideas, and as i don't use pencil crayon that often, they aren't what i always do (most of my work is oekaki, these days), so it would probably be best for you to try out a bunch of ideas, experiment, and see what works for you!