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Pinch back most seedlings when they've made three sets of true leaves. Just snip the top part of the tiny plant off with your fingernails to encourage bushy growth and more roots. Best to double-check your seed package to determine whether pinching is recommended.
Make sure your seedlings continue to have all the sun they need. Also keep the seedlings watered and weeded. Add an inch or two of organic mulch as soon as seedlings are up and growing.