I harden off my seedlings by taking them outside for an hour or so per day, first in the shade and then in the sun. I increase the amount of time they spend outside until finally they stay out all day (that might take a week).
I think getting used to the wind and the sun are the two things you're really looking for when hardening off seedlings. Keeping an eye on them and seeing how they respond give you clues as to whether they need more or less exposure to the elements. They'll either thrive (leave them there) or wither a bit (bring them in for recovery at that point).
