Recently I created a video course here on Tuts+ called Creative Vector Hair. It's an advanced course on how to render hair from scratch in vector. It will take you through five different projects and you'll end up with 19 different end results!
What I'd like to share with you today are some basic steps on how to create your own vector hairstyle from scratch. For the purpose of this tutorial, I'll be using a base portrait. In the course, you can download this file to work from as part of the course files should you wish to follow along. If not, feel free to download the image and sketch on top.

1. Map Out the Skull
When you're first approaching a new hairstyle, I'd highly recommend first mapping out the skull area of the portrait. This is so you know which areas would definitely have hair overlapping. So let's run through how you'd create a basic guide for the scalp.
Step 1
The first step is to establish where the hairline is. You could do this by following the original stock image or you could estimate where this is. To do the latter, remember that females will have a much rounder hairline compared to males, foreheads are about the same length as the nose, and hair needs to come in on the sides.

Step 2
Your next step is to draw out the remainder of the skull area. This can be tricky if you're following a stock image, as some images may have a large volume of hair which would not make it clear where the skull is.
To get around this, consider that the eyes are the half way vertical point of the head and use this as a guide to where the top of the skull is.

Step 3
Now that you've got your two basic lines created, you can then use these to create your hairstyle. Remember, whatever you create, this area must be covered in hair to create a more realistic look. Your lines don't have to be accurate—they're just a guide.
I've combined both of my lines and then reduced their opacity, so that they don't get in the way of my sketching.

2. How Volume Affects the Hair
Now that you have your guides, it's time to sketch out a design. So here are some tips for you when creating your own sketches. The example sketches I'm showing you are a bit crude but are drawn to illustrate a point.
Does the Hair Have a Parting?
If you're working on a hairstyle which has a parting in the hair, I suggest you draw the parting first and then build up your sketch from there. I'd even go as far as including this line within your guide for the skull.
When you have a parting in the hair, there is one main thing to consider: hair volume. If the parting is off centre, as shown below, then one side of the hair will have more volume than the other. The exception to this rule is if the hair is layered. Hair layering often avoids this imbalance of volume.

Consider if the portrait is looking straight on, i.e. your subject is not tilting his or her head. This will also affect the volume of hair due to gravity wanting to flatten the hair. So if you're using a parting and the head is tilted, consider on which side the hair would appear to have more volume due to gravity and the hair parting.
Does the Hair Have a Fringe/Bangs?
Forgive my British terminology here—in the UK we refer to hair which is cut to cover the forehead as a "fringe", but you may be more familiar with the term "bangs".
When creating a fringe, consider how much volume the fringe will have. Is it a full fringe like, say, Cleopatra, or just a thin sliver of hair covering the forehead? Based on this, you'll need to accommodate the volume of it by how far back the base line of the fringe will be.

Is the Hair Tied Back?
If the hair is tied back behind the head, there are a few things to consider. Firstly, the hair that covers the scalp will have some, but not a great deal of volume (of course this also depends on hair texture). So while you won't want to render a lot of hair on top, it won't be following the exact lines of your skull mapping. This being said, if the hair is pulled away from the face, you'll need to follow the exact line of the hairline guide.

The second thing to consider is the volume of the hair coming out of the ponytail. If it's pulled together with a band or scrunchie, then the base of the ponytail will be a lot thinner than the hair towards the end of the ponytail. This is because the hair is being held tight within the area of the band, and as it flows away from it, it's not as constricted.
Thirdly, if you're working with a lower ponytail, depending on the angle of the face to the viewer, you may see the bottom of the hair sagging at the top of the ponytail band. So keep this in mind.
Straight Hair and Curly Hair
Another thing to consider when working with the volume of hair is the hair's texture. If the hair is curly, then it will have a lot more volume than hair which is straight. Straight hair will lie flatter to the scalp, whereas tight curls will have a lot more lift at the roots and will have more volume.

How Long Is the Hair?
The length of the hair may influence the volume of the hair. If you've got very long hair, there won't be as much volume towards the top of the hair, as the weight of the hair will pull it down. This is especially so with long, straight hair.
On the flip side, if you've got shorter hair, there isn't as much hair to weigh it down, so there may be more hair sticking up and more random hair flying away from the scalp.

Sketch Away!
These are just a handful of things to consider when sketching hair. In my video course, Creative Vector Hair, you'll learn how to create a variety of hairstyles and what to consider in each one. From that, you'll learn how to render the hair in a variety of shades.
Why not put these tips into practice and upload your hair sketches in the comments below? I'd love to see your creations!