This probably won't be a helpful description but it basically comes from lots of experimentation. There might be underlying logic to the types of shapes I'll try, based on the planes or silhouettes of the part of the character I'm drawing, but I'd encourage anyone to simply indulge random whim! Any of my posted pieces were preceded by tons of discarded and reworked sketches.
"Hm, what if I did this" is the thought that get to run amok in digital art. "What if Mario's head was a giant square?" and then just try it and see. Most of the time it'll look bad, but you're also learning something every stroke you make, even if only subconsciously. And that growing knowledge will boost and steer ways to innovate next sketch!