In a post-apocalyptic setting, nothing is clean or pristine. Hairstyles tend toward the short, scruffy and utilitarian. While you’re designing your characters, ask yourself whether they have access to water for grooming, or even a comb. Do they care how their hair looks? Do they spend time to make it presentable?
To get started, draw the head and then determine the hairline. Placement varies; you can set it lower or higher on the forehead. For older characters, you can set it even farther back to give them a receding hairline.