Once you have the head done, it's time to add some hair. If it's not apparent enough from my earlier examples, I love drawing hair and making up crazy hairstyles. It's a lot of fun once you get the hang of it since the great thing about manga is that hairstyles don't have to make any sense.
After you finish drawing the head, I suggest you always draw the hair before doing the torso. Why? Because hair (especially poofy hair) will make the head appear larger. If you've already added a body without taking into account the hair, your head will end up looking way too big. This is better illustrated through the example below. Notice how the guy looks slightly skinnier in the second picture than in the first? It's because his hair causes his head to look bigger compared to the width of his torso. That's why it's better to have the hair done already before drawing the rest of the body.
Natural hair (without gel or products) is supposed to be light and flowy, so this is where long, confident, and fast strokes come in handy (See 1.3 - Strokes). Generally, when drawing hair, you want to draw a hair strand from the top to the bottom in the least amount of strokes possible. If you use too many strokes to draw one strand, the hair ends up looking fuzzy, brittle and dry. In some cases it may even look like fur.
I used long, quick strokes for the hair in the first drawing. They extend from the top of the head to the bottom of the hair, more or less. In the second one, I drew the hair using short, quick strokes. Notice how unnatural the hair looks in the regions I circled, compared to the same regions in the first drawing? I did exaggerate a little in some parts, but doesn't the hair look coarse even in places I didn't?
So where to start? First of all, it's very important to remember that heads aren't flat. You don't necessarily have to draw the top of the head in when making a face, but you do still need to keep in mind that there isa top, and it should be pretty round. Thus, the outline of the hair must be above the top of the head, otherwise you're going to end up with a caved-in head.
I generally like to start by drawing in the bangs (or whatever hair strands that are framing the face). Remember that being very flexible, hair will curve along the edges of the face even if it's straight. So remember to avoid drawing hair using only straight lines, as it will look unnaturally stiff, unless you are purposely going for that effect (as the popular manga-ka group CLAMPsometimes does). Once I'm done with this step, I continue by drawing the top of the head and making sure that it doesn't look too flat. And when that's out of the way, I just let myself go all out and allow my pencil to stream down the page naturally, stroke by stroke. If I feel that the top of the head is too high or too low, I readjust and redraw the rest.
There are two major ways to give shape to hair. One is to draw every strand individually and build up the shape strand by strand. The second method is to draw the outline of the hair shape and fill in the strands afterwards.
The method you want to use depends on the style you're going for. The first method (building strand by strand) makes hair look very realistic and flowy. The downside is that it's hard to control the shape since you can't really see the finished product and the overall shape until you're done. In fact, you'll probably need to keep on making readjustments at first to get the shape you have in mind. Shoujo manga artists often use this method (eg. CLAMP, Tanemura Arina, Hino Matsuri) because having individual strands also softens the whole atmosphere of the drawing.
Method 2 (drawing the outline and then filling it in) allows you to control the shape of the hair very easily since you're drawing it directly rather than building up to it. The downside is that the hair often ends up looking chunky and less realistic, but if that's what you're going for, then it's fine. Shounen manga artists usually use this method (eg. Kishimoto Masashi, Togashi Yoshihiro, Toriyama Akira), perhaps because they find it a quicker method and it makes the hair look less girly (I'm just speculating here). You don't have to stick to one style. I alternate between the two whenever I feel like it and sometimes go for something in between. It's up to the artist.