You should determine where/the direction your light source is coming from first and foremost when it comes to shading/highlighting and everything else usually comes easier.
For example, and this is very basic, if your light source is shining from above the subject it means the top of the head will be highlighted, but any hair over the face will be casting a shadow on the face, the nose will have a shadow, the chin will cast a shadow on the neck. Hair that overlaps might cast a shadow on hair that's underneath it, this goes for fabric as well such as folds and wrinkles. On the other hand, things that are raised up will be highlighted: cheekbones, tip and bridge of the nose normally, shoulders, top of the breasts, top of collarbones (the part that sticks out, where above might be shaded to show the skin that sinks in above them)
When I color things I generally try to avoid using burn and dodge as it can make things, skin, especially, look awkward or sickly. Its better to use your own judgement by taking the base color and finding a slightly darker or lighter shade that you think is best, imo. Also try not to blend or blur shadows too much or too far out because it could easily make your work look murky.