Just cover the small seeds, and thin or space to 15-20cm (6-8″). Crowded plants bloom more generously.
Note that the Baby’s Breath one associates with bridal bouquets is grown (mainly in Mexico) in very controlled settings, and harvested with precision just as the flower buds begin to open. This is the same flowering plant, but it is difficult to predict exactly when the plants will be bouquet-ready. To grow for summer weddings, direct sow large amounts at regular three week intervals starting in early spring when there is still a risk of frost. That would be mid-March here on the coast. Keep plants relatively crowded. This is the only way to ensure that at least some Baby’s Breath will be at its peak for your special event.