That’s right: the process of hardening off seedlings should start indoors first.
Make sure the area is free of wind and above 60°F. This could be under an eave, a covered porch, or a shady tree.
The next afternoon, put the seedlings outside for 3 hours in partial to full sun.
Bring the seedlings inside for the night.
Keep the potting soil moist, and bring the seedlings inside at the end of the day.