You can create your own brushes with different shapes, properties and dynamics, or download custom brushes from free or paid sites.
Create a black image on a white background (Which can be on the layer below). Go to Menu > Edit > Define Brush Preset. Name it and save it. The new brush is automatically selected as your current brush. Open Brush Settings to fine tune how the brush works.
Largest size to create brushes from is 5,000 pixels.
A simple brush can be used to brush in hair to hide a poorly selected edge or add extra detail. This adds further details to the page on Selecting and Masking Hair.
It is best to use a pressure sensitive pen for painting and have the Brush Settings set to give Shape Dynamics with Size Jitter set to Pen Pressure, and Flow Dynamics set to Pen Pressure. Then the hair will be denser and wider with more pressure. Saving this brush with a set Brush Diameter will be useful to use in future.
Using a mouse gives less options and always starts with a hard edged full thickness point, so always start near the scalp. Set the Shape Dynamics with Size Jitter set to Fade, and Length set to a suitable value usually smaller value. Set the Transfer with Flow Jitter set to Fade, and Length set to a the same value.
Using Rotate View (R) to rotate the canvas to the most comfortable angle for natural drawing helps you draw in hair more easily.
Hair Brushes can be used directly onto a blank layer after selecting a hair colour.
Hair Brushes can also be used to paint onto a mask, either with black to add to a mask but also painting in white to subtract from the mask.
In the Colour Dynamics settings, adding some Brightness Jitter will give different hair strokes different brightnesses and can look more natural. This is specially good for light hair.
Different hair brushes can be created for different types or hair, including beards.
Adding a small amount of Gaussian Blur also helps hide the extra sharpness of the added hair to match it to the image. However, painting over with the Blur Tool gives even more control as you can selectively blur different areas of hair to match how in or out of focus they are in the image.
Adding monochromatic Noise to the hair you painted on matches it better to existing hair, which will normally have some noise in the image.
The Pen Tool can also be used to define a path for the hair which the custom hair brushes follow. This gives even more editing possibilities for adding complex hair patterns. Phlearn Pro has a whole tutorial on this. (Subscription only).
How to Cut Out Hair in Photoshop by Phlearn ( Phlearn Pro tutorial series only available by subscription) was used as the inspiration for this section. It is my recommended resource for understanding how to create and use Hair Brushes and subscribers can download the hair brushes he has made.