NOTE: Unless stated otherwise, start each part on a new layer.

Part 1:

Using the pen-tool, outline your hair. For those of you who don't know how, or don't like using the pen-tool, feel free to use a small, hard round brush. Your outline doesn't have to be perfect, it just needs to be a 'guide', basically.

Part 2:

Pick your colours. It is possible to change the hair colour once it's complete using adjustment layers, but it saves some time if you pick your colours now. I'd recommend picking four or five shades of the same colour, ranging from very light to very dark.

In the example I'm using, I've chosen red as my main colour, so I will be using varying shades of red.

Now, using your darkest colour, fill in all the parts where you want your hair. This can be done using the paint bucket, or a hard round brush. If you use a brush, try to stay in-line with your outline.

Part 3:

Select your hair brush. You can find these brushes on sites like DeviantArt, simply type in 'hair brushes', and you should find a few. Please note: the brushes shouldn't be completed hair pieces, they should basically look like a bunch of round brushes in one brush, if that makes sense?

Part 4:

Using a lighter shade of your chosen colour, brush in a few random streaks all over your hair. It should look something like my example below.

Part 5:

Now, duplicate the layers of step 2 and 4 (the main colour and the streaked colour), and merge your duplicated layers. Make sure NOT to duplicate your outline. Hide your extra layers and work on your duplicated layer.

Using the same brush your used in part 4, use your smudge tool on 100% strength and begin to blend your lighter streaks into your darker hair colour.

Part 6:

Reduce your smudge strength to about 80% and begin to smudge your hair out of your outline. Ensure that you've smudged all of the bottom and side parts, leaving only the top part of your hair. It should look something like my example, although feel free to deviate if you want to.

You can now hide/delete your outline layer.

Part 7:

Select another hair brush, and select your darkest colour (this would be the very first colour you worked with), use the pen-tool to start brushing in streaks. Make sure your stimulate pressure box is ticked.

Continue this until all of your hair is covered.

Step 8:

Select another hair brush, and a lighter colour. Now repeat step 7. Keep in mind, the more time you spend on it the better it will look.

Step 9:

Now duplicate all of your layers (only the ones you are working with), and merge ONLY your duplicated layers. Note that is very important that you don't merge the originals as well.

Now hide your original layers, and work on your duplicated layer.

Using a hair brush with the smudge tool, begin to smudge hair into the face, this should be done using about 80% strength. Once this is completed, unhide your bottom layers.

Step 10:

Using a small hair brush tool, start to brush in the final details with the BRUSH tool, not the smudge tool. Note, you can use a darker or lighter shade.

Step 11:

Click on the filter option, and then on oil paint.

Adjust the sliders to your liking. Keep in mind, it should only smoothen your hair, not add details.

Step 12:

This is the end result of the brushing. 

Step 13:

Now you can play around with adjustment layers (starting with curve) to change the look and make it more realistic. Down below I have a few examples of what it CAN look like with the right adjustment layers.

Although this is completely based on your taste, you can do anything with it from here on out. Also, this was a pretty rushed tutorial. It can look really good if you spend some time on it!

I hope this tutorial helped you! If you have any questions, feel free to ask them in the comments, or PM me. I'll try my best to answer them!

As a bonus, I've added a PNG of the finished product in brown colouring, if you use it, please credit me!

Do you want more tutorials? If so, is there anything specific you'd like me to do a tutorial on?