Select transplants carefully. Look for sturdy rather than large/tall seedlings. Transplants should have at least three pairs of dark-green leaves. They should be well rooted, but not pot bound. (I tap one of  the starts out of the tray to check the roots). Ideally the roots should  just be reaching  the sides of their container. Most store-bought seedlings will benefit from a week of "hardening off" before planting, as they may be moving from a very warm greenhouse to your chilly damp outdoor garden. To help them through this phase, put plants outdoors in progressively brighter light each day, but bring them back in at night. Continue for about a week.   When ready to plant, scoop out a hole in your garden soil about 4 inches deep and mix in a small amount of organic fertilizer into the soil. Next, I like to set the transplant container into water up to the soil level and wait for the air bubbles to escape and the soil to loosen from the sides of the container. Then I carefully tip out the seedling, touching the plant as little as possible. Try to handle them from the seed ball or leaves, not the stem. Plant them up to their first two leaves so the plan won't fall over.  Press the soil gently around the plant, but don't squash the root ball below.  Water well with tepid water. Give them a bit of special care for the first week or so.

Much of this information is courtesy of fellow Community Gardener Dave Robison