So said Severus Snape.
But there is much better advice on how to teach heresy from Carl "Mad-Eye Moody" Trueman over at Ref 21.
Here's the introduction:
Teaching heresy is surely one of the most important things that I have to do in my classes at Seminary. Friends will at this point throw up their hands in horror; enemies will smile smugly to themselves and mutter `I told you so!’; but it is true. Teaching heresy is, for me, a crucial part of my responsibility as a professor. The reason, of course, is simple: in order to know what orthodoxy is, one needs to know what heresy is. Indeed, a study of the creedal development of orthodoxy, particularly in the early church, demonstrates time and again that the defining of orthodoxy and the defining of heresy is something which the church does simultaneously. This is hardly surprising: creeds establish boundaries, and so the establishment of creedal orthodoxy is one and the same act as the establishment of heresy.
So far, so obvious. That is the "why" question answered. The "how" question is a little more complicated...
Well go and read the rest. The ending is good too:
...those called to be teachers in the church need a solid grasp of orthodoxy; and that demands by its very nature a solid grasp of heresy. That is why I teach heresy in my classes, and why I make sure I do justice to the legitimacy of the questions which underlie virtually every heresy of which I can think; for it is only then that I can truly explain orthodoxy to my students. And I also get a perverse pleasure from using heresy to do that which heretics most despise: promote sound, biblical, historic orthodoxy.