Scrape off any dead stuff like dead algae or corals from the rock. Keep as little dieoff as possible in the tank...
Next put the rock in the tank and wait. As things die they will cause your ammonia levels to rise. Do a few water changes along the way so that you minimise the toxic effect that this has.... When things die they release ammonia. Then the ammonia poisons the things that are still alive and then they die which adds more ammonia... Change the water to mimise this effect.
Rock is "cured" when it is no longer experienced mass dieoffs from the lifeforms within it. This takes about a month.
The best way to cure live rock is to do it in their own dedicated tank. If you try to cure it in your main display tank you may risk crashing your whole system and killing everything in it. If you DO decide to cure LR in the main tank, stay on top of your water changes. Keep your levels down...
SWF.com sells precured LR... You will not get much dieoff from rock bought from them. However it's still a good idea to let the rock sit in its own tank for about a month before you put it in your main tank.