What I will usually do is, at first, put my plants outside for 2-4 hours. After a few days of that, I will start the schedule back over, but in direct sunlight for the same amount of time. After about 2-3 weeks, I am able to leave them outside as long as I want.
1 hour the 1st day in the shade, 2nd day 1 hour in the sun, 3rd day 2 hours, 4th day 3 hours, 5th day 5 hours, 6th day 7-8 hours, 7th day 12 hours, 8th day leave out all day. I have always had good luck hardening off this way.