They don't require that same incubator environment that seed germination did. Fertilization Seeds don't need food while germinating because they contain their own food, just like an egg. But seedlings will rely on you, just as chicks do on their mother hen. Start giving them some liquid nutrition. Any good all-purpose fertilizer such as Rapid Grow or Miracle Gro will do fine.
Seedlings are delicate - begin by applying fertilizer only at a half strength concentration.
Feed seedlings once or twice a week. Thinning Is Critical Please don't neglect thinning out your seedlings, keeping only the best specimens. Do this by using a small scissors and snipping off the weaker and excess seedlings at soil level. Don't just yank them out as that disturbs the roots and soil of the plants you're planning to keep. Thinning is a critically important step.