It's pretty straight forward, just put dirt in the egg carton and you're good to go! I'd recommend only using the cardboard type, as this will allow for proper drainage for your soil. Another thing I would recommend, because these dry out so easily, is to put them in a tupperware or other plastic receptacle and pour the water in it, and let it soak up through the bottom of the egg carton. Seeds seem to like indirect moisture better than having it dumped straight onto the soil, and this seems to work better with this method.
We didn't start many plants in these this year, as I've been pretty obsessed with my Newspaper Seed Starters, but the ones we did are looking ok. I'm going to have to either transplant them or get them in the ground soon, though, in order to keep them alive.
Have you done any green seed starting this year? If so, let me know! I would love to hear about all your trials, even if they're failures! :)