The process is pretty simple. Over the course of a week you expose them to the outdoors for a few hours a day and then extend that exposure until they've experienced a full 24-hour period outside. At that point they're ready to transplant.
Hardening off needs to be done for all plants before you place them in soil outside. Plants you buy at the nursery or garden center may look strong and healthy but they started growing in a greenhouse and have been protected their entire lives. If you take them from the comfort of a store or covered growing facility and plop them in your garden bed, it's a bit like you getting off the warm couch on a cold winter night and walking outside barefoot. It's a sudden shock and unpleasant enough to cause important parts to shrivel.
Plants that you started from seed are probably even more coddled. They've had perfect light, temperature and water that you oversaw personally. Transplant them without hardening off and all of your efforts will be wasted. Transplant your seedlings without this effort and they probably won't survive.
Begin hardening off by selecting a protected area outside. A deck or patio near the house, a spot under a tree, or a shady location next to a fence are all good. You want the area to be relatively free from wind and harsh sun. If you can begin on an overcast day, you get bonus points. On that first day put the plants out for just a couple hours. Make sure they're well watered; you don't want them drying out. You're just trying to show the plants what they can expect in the days ahead. After the brief exposure, bring them back inside to the warm, well-lighted area they came from.
For the next couple of days put them in the same protected area and leave them out for a little longer each time, increasing exposure to the sun gradually. By the fourth day you can put them in a spot that isn't so protected and let them soak up most of the late morning and early afternoon sun. Mild wind will help them get stronger. Keep them watered and don't fertilize them.
About day five, start leaving them out all day with exposure to full sun and reduce the amount of water you give them. Start letting the soil dry a little, but don't let it dry out completely. This is really the point that you're telling them to toughen up. Life is going to be hot and dry and they better get used to it.
Take a look at the weather forecast and if the night temperatures will be above freezing and not too windy, plan to leave them out overnight on day six or seven. If they look strong in the morning, they're ready for life in the big outdoors. Don't water them the day that you plan to put them in the ground unless the soil in the pot is bone dry. You want them to be a little parched and ready for water. They'll get that needed drink after you transplant them in their new bed.
If after a week the plants look a little ragged in the morning or if the forecast calls for extreme weather, like hot and windy or cold and snowy, go ahead and continue the hardening off process for a few more days before planting. You don't want their first day in the ground to be too harsh.
The whole process takes about seven to ten days normally. Bigger plants that may have had outside exposure at the nursery or garden shop may require less time to harden off, but you should still do it so they acclimate to your garden. Smaller and more tender plants may need a more gradual hardening off that will take longer. You have to assess the condition of your plants each day and determine if they're ready for extra exposure on the next day.
After hardening off the plants are ready to transplant in their new location. They'll be able to handle sun, wind, and weather right away. They're not invincible though. In the first few days if a sudden freeze or hail storm is in the forecast, cover them with a blanket or plastic cover; they may not be strong enough to survive.
Hardening off is the same for all plants. Annuals and perennials and flowers and vegetables will all benefit from the effort you take on their behalf. It can be a lot of work if you have a lot of plants, but it's necessary. Scrimping on hardening off may keep your plant from growing up to its potential.