The Shadow: This beard works well for men that can grow facial hair quickly and have a good, strong chin. You want to keep the beard tidy, give it shape and make sure it doesn't look sloppy or messy. Trim just below the jaw line to help outline your jaw and use good trimmer everyday to get the right look.
A close relative to this style is the stubble look, ever so slightly longer, in which you also need to maintain good grooming habits to prevent looking like a bum. For men with fair hair and men with light or medium facial hair growth, this will be more attainable than a shadow, since you probably hardly get that effect.
Bottom line is that you need to not shave for 1-3 days, then make sure you carefully trim it daily for a shadow and once every 2 or 3 days for a stubble look.