The starting point is of course whether you think it can or cannot exist. If you think it can, I would outline Hart, Kelsen or Raz's view of positivism and then say how it is accurate in explaining how law functions, then also incorporate criticisms of Fuller's inner morality of law (i.e. his 8 principles are more guidances for good legislation and not moral viewpoints as such). If you think it can't, focus on Fuller, Finnis and Dworkin's criticisms of positivism.