Cging can be learned through decent knowledge of the graphic program's tools and of course how to use them decently enough. Watch a good amount of speed paint and read some shading tutorials and you're pretty well off. The hardest part is putting the information into practice. This is where you need the time and effort put into it to show progress.

Eddieblogspot has a nice vast selection of them, just get a few and you too if you put the time and effort into it, you'll get some decent results. Youtube, deviantart, pixiv all have tuts on all sorts of mediums of drawing if computer graphic isn't your cup of tea. Proper keyword is important :P