In https://gardening.stackexchange.com/a/421/138, the answer mentions "Remember to harden off any indoor plants for a few days before transferring them outdoors." I assume this is something you would need to do to help them survive in the colder, more variable climate outside. We're trying to grow tomatoes and peppers, both of which are supposed to be started indoors if I understand correctly.
How do I do that?
For what it's worth, we live in central Minnesota, so that's the climate they'll be moving out into.
Here's what I do over the course of a week before I transplant to the garden. This is sort of the "ideal" plan, it never goes exactly this way -- variations are ok.
On the first day, which is ideally overcast and not windy, set the plants outside for a couple of hours. If it is sunny, I put them out in a spot where they'll be shaded for those two hours. If it is windy, I put them in a spot where they are sheltered from the wind, or I leave them inside for an extra day if I can't provide sufficient shelter.
On the second day, same shelter consideration, but I put them out for an extra couple of hours. (All morning, or all afternoon.) Be careful to check the soil moisture level -- pots can dry out fast on a warm, dry, sunny or windy day.
On the third day, I'll leave them out most of the day, with the same shelter considerations. If it is sunny, I'll move them at midday (because my morning shady spot is sunny in the afternoon).
On the fourth day, they're out most of the day. I still shelter from wind, but I let them get a bit of direct sunlight.
On the fifth day, basically the same, letting them stay out later in the evening.
On the sixth day, as long as we aren't expecting rain overnight, I leave them out overnight, but protected from marauding animals (i.e. move them to inside the garden fence).
On the seventh day, they get transplanted.
Watch out for rain! My "hardening off area" is under the eaves of my house. I work from home, so I can scurry outside and rescue the plants from rain if we get a surprise storm. An unexpected downpour can quickly water log your planting pots.
Slowly get them used to their new environment. For tomatoes, This would mean placing the plants outside for an increasing amount of time each day until they are completely used to it. Start When the frost is past outside and the average outside low temperature stays above 45 degrees Fahrenheit. I usually put them out for an hour the first and second days, three hours the third day, five hours the fourth day, and keep going like that until it stays out from sunrise to sunset. Then plant it.