Create a new layer with Layer > New > Layer and using the Brush tool again set to 100 per cent Opacity/100 per cent Flow, draw an erratic black scribble across the page from left to right.
Scan the character art in at 300dpi (It’s much easier to scale high- quality art down than scale low-quality art up) and desaturate in Image > Adjustments > Desaturate and then Image > Adjustments > Brightness/Contrast adjusting the contrast until the image is black and white. The image is desaturated so that when you increase the brightness/contrast values you are presented with a pure black and white image.
With the Brush tool set to 100 per cent Opacity/100 per cent Flow, use it to neaten up the edges and fill in the rougher areas of black. Draw into the image further to define the hair in the beard and eyebrows. For this sort of accurate painting a Wacom graphics tablet is essential for complete control of the brushstrokes. This could have been done in Illustrator as it’s only a black and white image, but I find Photoshop’s Brush tool more organic and intuitive than Illustrator’s pen tool.