That's the basics, but to add a
little more detail, if you still need it:
Basically it's a bookcase deep enough to hold whatever tub you're going to use to house them in. You can choose to have the tubs be closed by having the clearance between the top of the tub and the bottom of the next shelf close enough that the snakes can't get out, or you can put lids on the tubs. Usually a heating element of some kind is put under each level of tubs at the back. This can be heating cable, flexwatt, or whatever preference you might have.
As far as ideas go, there's a great thread over in the photos section about exactly this topic:
http://cornsnakes.com/forums/showthread.php?t=63536
I'm sure I've seen a few other threads around here as well.