Summarize the following:
Monitor the growth of the seedlings closely. Don’t let the seedlings get too big, as more leaves will form, which can cause crowding. It's easiest to wait until the seedlings are tall enough to grasp. Seed leaves are replaced by true leaves as the seedlings grow. Look for leaves that match the adults, as seed leaves are more rounded and simpler than true leaves. Waiting for the true leaves to appear helps you pinpoint the healthiest seedlings to keep. Thinned seedlings have a greater survival chance if the weather is not too hot or cold. Moisten the soil with a hose or a watering can if it's too dry. It's easier to pull up roots from damp soil. Only start thinning out when the temperature is cooler. The remaining seedlings need time to adjust to the extra air and light before the increased sunlight exposure during the next day. This prevents the seedlings from getting stressed and helps them to grow into healthy adults. Look for the seedlings that appear bright, healthy, and disease-free. Use thin wooden sticks like toothpicks or popsicle sticks to mark these seedlings. Check that there is only 1 seedling per cell or that you have followed the recommended spacing. Taller seedlings aren’t always stronger. Avoid choosing tall and spindly seedlings, as these may not become healthy adults.

Summary:
Thin the seedlings when they are 2–3 inches (5.1–7.6 cm) tall. Look for 1-2 sets of true leaves on each seedling. Pick a mild day when the soil is damp. Wait until the evening to give the seedlings a good chance to adjust. Locate the strongest and healthiest seedlings to keep.