Starting flower seeds indoors is necessary for certain flower varieties that need a long growing season such as petunias, lobelia, marigolds, stock, snapdragons, ageratum and dusty miller, says Hume Seeds. Not all seeds need to be started indoors, so look at the packaging of the seed before starting the seed. The University of Minnesota Extension recommends buying seeds as fresh as possible to prevent disappointing results.
Fill your seed trays with commercial potting soil and sow the flower seeds according to the instructions on the seed packet. Different flower varieties require different depths in the soil. The University of Minnesota suggests planting the seed at a depth no more than four times the size of the seed.
Water the seed so that the soil is moist and place the seed tray into a window that receives bright light with a temperature i between 65 and 75 degrees. suggests Hume Seeds. If the temperature inside the house is less than that, place a clear plastic bag or a humidity dome over the plant to start the germination process. Remove the bag once the seeds begin to germinate. Turn the plants regularly so that they do not bend towards the light.
Transplant the seedlings once they reach a height of one to two inches by digging the seedling up with a sharp knife, holding it by the leaves and not the stem, and placing the seedling in a new pot with fresh soil at the same depth it was at germination.