Answer: Congratulations on your new "babies"! First of all, be sure your seedlings have plenty of light. A fluorescent light just a few inches above the growing tips is best. As soon as your seedlings develop their first or second set of true leaves (the first leaves that show are called "seed leaves"), thin the plants down to one plant per cell, if your kit uses 6-packs or similar containers. (Each seedling should have it's one 1 to 1 1/2" container.) Choose the stockiest, strongest looking seedlings, and snip away the others with a pair of scissors right at the soil line.
Here is an article from National Gardening magazine:
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