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. Somewhere with a light breeze is also good for them at this point.
Put them someplace warm with a soft breeze, if possible, and make sure they don’t sit dry and wilted for a prolonged period of time.
Bring them outdoors earlier in the day so they experience some cooler temperatures, and give them 5 to 6 full hours of direct sun.