The camera lenses are simply glued into the housing with adhesive, so essentially what you'll be doing is picking the pieces out of the bezel if it's shattered, or just pry it out with something like an X-acto knife if it's just got a crack. The use of an iOpener/heat gun/hair dryer to apply heat to the glass will help soften the adhesive and make the glass easier to remove.
Once you've got the glass out, you'll need to clean out the adhesive; scrape it out as best you can then clean it with Q-tips and isopropyl alcohol (preferably 90% or higher, not the 70% stuff we all have around the house).
Once everything's shiny and clean, apply the adhesive that comes with the replacement lens then put in the lens. A little heat followed by some weight on it will help seal it good against liquid ingress.
So basically it's pretty much what you'd expect when you have a cracked piece of glass that's just held in by glue. Obviously you'll want to try not to get anything on the camera lens once it's exposed, and if anything does fall in, make sure you get it out or you'll be cursing yourself as you disassemble the phone to take the camera out and clean it again, lol.
That's it; it's all pretty straight-forward, so good luck and let us know how it goes!