Quick and easy starting point for stubble and shaved heads
First create a flesh colored background.
Fill the stubble area with the color the beard is supposed to have.
Create a mask and add noise to mask at 400.
Apply mask, create a new mask and add noise at 200 again.
Using brush on the mask, paint the edges to feathered. Change blending mode to multiply and opacity to around 20%.
Apply mask. Motion blur at 90° and value of 1 to make it more hairlike.
Try painting the fill layer at the beginning and before collapsing the final mask to give the hair more volume. This method needs serious tweaking depending on the resolution of your texture and is best used in conjunction with some hand painted hairs.