i think it's difficult because of the whole "who do i begin" specially with so many approaches and styles available, the style I used in this video is very easy for beginers, if you look for tutorials on google images and how to shade objects you are always going to find the typical ball with gradient light on one side and a little dark shade on the other lower side and things like that, basically what i am doing is using airbrush to give volumes to geometric shapes, and lasso to restrict where i want and how i want the volume to rise that's it, but that is the basic idea here.

I really hope this video gives you more hints, the tools I used are the same in photoshop, sai, clip or krita so everything should apply.