I've actually never done it myself, but there is tons of info if you do a search for it on this forum. Basically, if you haven't cycled your tank yet you can just throw it in the tank and it will kick off the cycle and become cured (get rid of all the dieoff and bad stuff). But your tank is established so you'll need to cure it yourself.
Basically get a rubbermaid or some sort of tub with some water, a heater, and a powerhead and let the rock sit there for a while. How long you leave it depends on how much rock you have I believe. It could be anywhere from 2 weeks to 6 weeks. When you are done, the rock shouldn't smell and it should not be producing ammonia.
I didn't explain the particulars very well, but that is the basic process. Maybe other people can help out a bit more. Do a search. I'm sure someone has written this up more eloquently than I have. Good luck.