I have a lean-to greenhouse where I have placed growing trays on top of a heating mat. The mat is made especially for this purpose and it heats the soil from underneath to a temperature that fosters good growth in spite of cold weather outside . I make sure the soil is very wet and the plastic dome cover keeps that moisture inside. It doesn't need watering very often and I can tell if it does when the condensation inside the cover lessens.
These broccoli seedlings that were sprouted indoors took quickly to this moist warmed soil and sunshine. I took the cover off just for the photo. Keep the cover on continually up until the seedlings touch the cover. At that point, you will want to gradually keep the cover off to harden the plants and allow them to grow freely without it.