First you need a styrofoam egg carton. You'll cut the lid of the carton off and set it aside for now. It's going to become your watering tray.
Next you will need make a little hole in the bottom of each of the egg spots. I used a nail that I had heated up and sort of "melted" the holes. Be careful not to get them too big.
Now you want to cut one or two of those "bumps" in the middle of the egg carton. This is where you will be watering your seedlings. I've been using a funnel to water mine and it works great.
Many of my egg cartons are made differently. This one just has that one "high ridge" in the middle. Some of the other ones had a bump on each side. It really doesn't matter though. As long as you get the holes cut.
Set the piece on top of the lid you have put aside. I wrapped the whole thing in tinfoil to help it all stay together. That way if I accidentally overwater it doesn't leak so bad. And there you have it. Add some dirt and your favorite seeds. I like this because it waters the plants from the bottom and therefore its not watered unevenly.
Once you get your seeds planted you might want to cover it with some saran wrap. It will help create a good atmosphere for them.
So there it is. My new seed starter trays. Now I just need to find some more egg cartons!