So an artist has drawn a face, yet the picture has no hair. What to do? Well bald is beautiful, but some people (especially female characters), prefer NOT to be bald. Today, curly hair is in order.
Draw out the outline of where hair should be placed. This will help with hair strand placement.
Add your dark tones to your hair. Yes, it may look like funny streamers, but trust me. These are your shadows.
Add your midtones to your hair and make sure to overlap your dark tones to make those tones darker.
Take your eraser pencil and "lift" sections of your hair for light sourcing and for adding depth to hair strands. If you feel you lifted too much dark from your hair, you can go back and draw more dark and mid-tones strands. You can repeat steps 2-4 as many times as you like until you achieve the effect you want. Remember, you can also take your tortillion and smooth and blend hair until you achieve the look you want.
Add "flyaways" (loose strands) to your hair. No one's hair is perfectly coiffed, there is always flyaways. Make sure you indicate that in your image as well.
The midtone part is the hardest and most time consuming part of the curly hair process because you have to draw each strand of hair separately in order to make it realistic looking. With straight hair, you can do large section of hair and add strand lines later. Unfortunately, curly hair literally is a strand by strand process. Take your time and have patience!