When your seeds germinate indoors and break the surface they are ready for the outdoor elements but they lose that ability as they spend weeks growing 'oh so comfortably' indoors. They have to be reintroduced to Nature. I can't give you an exact way to do this but to say... slowly over a one week period. Too many factors come into play such as; plant variety, temperature, time of day, amount of sun that day, the wind and day night temperature fluctuations.
This video discusses two ways to acclimate your plants. The way I recommend is to give your seed starts time outside 2,3,4 times a week, during nonfreezing temperatures, for 15 to 30 minutes as soon as they germinate. This exposure will make for an easy transition outdoors. The benefit is they will already be 'hardened-off' and the exposure to the sun will benefit their growth. The disadvantage to this is the possibility of insects hitching a ride back into your grow area. The second method is to transition them incrementally to the outdoors ,slowly over time, once the reach transplant size.
Good Luck with Your Garden,
Gary (The Rusted Garden)
Join My New YouTube Channel Just for NEW Gardeners: My First Vegetable Garden
Join My FB Garden Group: Grow It Cook It Eat It:Grow It Cook It Eat It