Don't! Simply don't play this hero. He is useless. Utterly useless. If you have ever lost playing against Amumu it's because you suck. He is good at nothing. He does no damage at all, there is no reason why anyone ever should attack him, therefore he is also a bad tank.
The only good thing about despair is the animation. It's what your team is doing when they see that they have Amumu on their team.
if you have no idea how to play some hero, then it doesn't mean it sucks