Prepping diff rings

[fearo]

So the time has come to renew my madrat's diff. I have all new parts, but i've read numerous times that it's better the prep the diff rings a little by sanding them.

Now, I have some 800 and 1200 grit sand paper, and my question is, how do I do this? How long do I need to sand each side, in what motion, and when do I know that they're ready?