Doing digital art, and shading it are two seperate skills all together as far as I'm concerned. Doing digital art by itself isn't hard. Lineart, inking and colouring are very basic things which you can learn in a matter of days.
In a picture, good inking takes long to do. But shading by far takes the longest and is the most anoying if you don't have a good idea of what you want to get. Learning how to shade a picture digitaly is a matter of experimenting and finding the tools you like best for it. I for one shade my pictures, using a new layer, putting in on multiply and using a soft brush. While others have entirely diffrent methods.
So I'd say, google around for some basic tutorials and get the hang of basic colouring first before worieng about shading. You need to crawl before you can walk
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