Starting vegetables and flower plants from seed is more cost effective than purchasing nursery seedlings. You also have a greater choice of plants when you choose to plant from seed. Most seeds don't require light in order to germinate as light isn't required until leaves are present. Most seeds do require warmth to germinate, which there isn't enough of if you just set the seeds in a sunny window. Seedling heat mats are available that allow you to heat the soil, where the seeds need it most, to the best temperature for germination.
Fill seedling trays or pots with potting mixture. Water each tray until the mix is evenly moist throughout.
Sow seeds in the potting mixture, following seed envelope recommendations for depth and spacing. Cover the trays with plastic wrap to maintain moisture during the germination process.
Set the trays on top of the seedling heat mat. Choose a location for the mat and trays where it won't be disturbed during germination.
Plug the heat mat in and set it to the proper temperature for the type of seeds you are starting (see Resources). Most seeds require temperatures of 70 F for germination, though it differs by plant variety.
Remove the plastic wrap once sprouts appear and relocate the tray and heat mat to a sunny window or under a grow light. Lower the temperature on the heat mat 5 F to encourage stocky plant growth.