Go for an unkempt look. Grunge is defined by a messy, I-don’t-care look that combines punk styles with working-class clothing. If you want to dress grunge, you will have to give up any value you might put into matching or looking too clean.[1]
Search the internet for pictures of some of the most famous grungers and grunge band members--Kurt Cobain, Courtney Love, Eddie Vedder, Chris Cornell, William DuVall (of Alice in Chains) etc.