What matters most for starting seeds indoors?

Light, warmth, and moisture. Use a seed-starting mix in clean containers with drainage, plant at the depth on the packet, and water from the bottom. Cover with a clear dome until sprouting. Heat helps germination, but once seedlings appear, they need bright light immediately—a cheap LED shop light two inches above the tops works better than a south-facing window. Keep soil moist, give 14–16 hours of light daily, and transplant after two sets of true leaves.