Its pretty complicated... but I make a new layer, draw (rather messy I might add) in a really dark color, for this picture I used red, then lower the layers' opacity so that its kinda see-through, then take the smudge tool and make it look like the edges are shading fur, I cant stand smooth shading on animals because technically they're furry all over! then for this specific picture, I made a new layer and used a one pixel brush with bright bright yellow and went over the edges of the lineart really thin where the highlights would be, and then again with pure white. usually though, I just use the eraser tool for that bit.
And so is this drawing