Start by spreading potting soil mix in a low, flat tray. Potting soil can be purchased at any garden center, please purchase the plain, cheap potting soil without the added fertilizers. Stay away from any pre-fertilized potting soils such as the Miracle-Gro type brands. Sprinkle your dahlia seed over the top of the soil. Then lightly cover with potting soil. Lightly water so that soil is damp and continue to water lightly keeping the soil damp during sprouting. Sprouting should occur within a 10 to 25 day period. DO NOT OVER WATER, OR KEEP SOIL SOAKING WET. Start them in 70+ degree temperatures and a brightly lit area. They should be kept in these conditions throughout germination, they do not need any additional light, other than what Mother Nature gives us during the daylight hours. Keep in mind that unlike vegetable seed, dahlia seed will not all sprout the same day; germination will occur over several days. The usual success rate in dahlia seeds is 50-75% success.