Make sure they get plenty of light (see "Do I need special lights") and fertilize weekly (see" I'm new to starting seeds...?). When they're big enough to handle,usually when they have 2-3 sets of leaves, it's time to transplant them to individual containers,or space them out if you have them in a community flat. Those sown in individual containers should be thinned to the strongest seedling by snipping off the extras with scissors.

Now that damping -off is less of a risk, you may want to use a part soil mix for "potting on", 1 part sterile potting soil to 2 parts vermiculite or perlite and /or sphagnum or peat, for the sake of economy.

If you need to transplant,very carefully dig up the plants, with a knife or wooden label,etc.,taking care to get under the roots. Separate them gently, handling them by the leaves rather than the easily-damaged stems . I often use a pencil to help untangle the roots, using the same pencil to poke a hole in the soil ,making it deep enough that the roots can dangle and the seedling can be placed at the same depth it was growing previously. Firm the soil around the roots, and water well. Repeat until you're cross-eyed,then start again the next day.....

Weather permitting ,seedlings may be transplanted outdoors at this stage ,or grown on indoors for larger transplants. Before planting out, seedlings or young plants must be "hardened off" over a period of 10-14 days. Begin by putting plants out in shade, giving some shelter from wind, and gradually exposing them to sun, maybe an hour at first,increasing an hour each day. Cut back on watering,but don't let them go completely dry.

Once they're hardened off, they're ready to go outside permanently (where they belong!!). If possible,pick a cloudy day . Again, try to handle the roots with care,and set the plant at the same depth it was growing (though tomatoes benefit by being planted deeper,roots will form along the stem)firm the soil around the roots and water well, a dilute solution of water-soluble 15-30-15(a teaspoon to a gallon of water) will help stimulate the roots. When all your plants are in the ground, fix yourself a tall,cool one and celebrate !