When drawing portrait with a pencil, I find it challenging to get realistic-looking hair. You may not want to draw individual strands of hair. If you’re not careful, the person may end up looking like Gollum. I follow an approach of first putting down a base layer by using a soft pencil to fill out the shape of the hair and blending stumps to create a uniform appearance. The blending stumps are used to effect a smooth, gradual transition between light and dark which is more realistic. I’d then draw the shadows and shades in the hair and blend some more. The highlights can be brought out with a kneaded eraser. The process is quite time-consuming, but the results can be more convincing.