From Burpee:

Take your seedlings to a protected location outside for one hour for the first day,” she said.

Do this each day for a week. Add one hour for each day of the process. By the end of the week, you’ll be at 7 hours and the plants will be ready to be transplanted.

While inside, seedling stems haven’t been exposed to winds. Plants, like us, need to start our workouts and gradually increase the intensity to become strong. So early on in the hardening off process, provide seedlings shelter.

Don’t put them in direct sun. Don’t put them in a windy location.

References:

How to Harden Off Seedlings - Gardening Tips and Advice from Burpee.com http://bit.ly/XCgb8C