I can definitely give an example on how I personal go about it. So here's a simple sketch I made.
It looks pretty good with details so far, but I want to add more. Now, I could definitely just add more lines here and there, especially for her hair. But in this case, I'm gonna go with coloring in the strands. I start off by adding some color to the hair.
Then, with a darker color, I begin to add in a bit of shading. I know exactly where my light source will be, so I start from there. You'll begin to see that I didn't decide to color in all of the sketch. Yet, the parts I did color helped to enhance it!
And, for a little bit of somethin', somethin', I add in lighter and darker shades of the same color to make the hair pop!
And there we go!
Sometimes, less is more when it comes to details. You can definitely start out with simple lineart and use other elements to make it pop. In my case, I used one color (in different shades), and the element of a light source to turn my simple doodle into something more