Prepare flats for sowing by spreading about half an inch of coarse sand or unmilled sphagnum moss over the bottom. Then fill to within a quarter-inch of the top with the medium you have chosen. Place the flat in a vessel of water until enough moisture is drawn up by capillary action to moisten the top soil. This will help to firm the medium into a flat bed for sowing and also make it properly moist but not sopping wet. Then fill to the top of the flat with more of the medium which has been screened through a fine sieve or a piece of quarter-inch hardware cloth. With a trowel, the edge of a ruler, or a piece of lath, mark off planting rows, about 2 inches apart. Sow the seeds shallowly in the rows, covering them with sieved soil to a depth of about twice their thickness, no more. Pat the rows lightly to firm them.
It is easier to broadcast the very fine seeds of petunias, snapdragons, begonias, lobelias, gloxinias and other gesneriads over the whole surface of the flat than to sow in rows, and these fine ones need no covering of soil. Or you can buy "pelleted" seeds of the very tiny kinds. These have a coating that protects from disease and also contains food. Pelleted seeds germinate quickly and, being larger, are easier to sow thinly, thus avoiding crowded bunched seedlings. They require special handling, so follow the directions that come with them.
Large seeds, such as those of nasturtiums, or the hard seeds of sweet-peas and morning-glories, seem to germinate more readily if soaked overnight in warm water before planting. You can nick the hard ones with a sharp knife, but I never found this very easy to do. You may prefer to plant larger seeds in peat pots filled with your medium. Insert one or two seeds per pot, water well, and stand the pots on a shelf above the heater to give bottom heat which promotes germination. When seedlings are well developed, transfer to a bench of soil, to standard pots, or plant directly in the garden with no intervening transplanting. Peat pots provide some nourishment and usually disintegrate in the bench or in the ground when roots penetrate the pot walls.
If you prefer, you can sow all your seeds in standard pots, bulb-pans, glass kitchen casseroles, or even in egg shells. Prepare pots by covering each drainage hole with an arched piece of clay pot ("crock" or "pot-shard," as these pieces are sometimes called) to prevent soil from washing out. To keep soil sweet in deep pots, spread a little horticultural charcoal over the bottom before filling in the planting medium.
As you finish with each packet of seed, label the flat or pot. Write the name of the plant and the date of sowing on each marker, also something about culture if there is room. Use a waterproof marking pencil so your identification won't disappear. You will be surprised how much alike seedlings look until the first true leaves (usually the second set of leaves) appear. If you sow more than one kind of seed in a flat, label each row.
Water the sowings lightly with a hand bulb-spray or mist bottle. Then cover the flats with a piece of glass to retain humidity, and spread newspaper over the glass, since many seeds seem to germinate best in darkness. Or slip the well-watered flat into a large plastic bag; or cover the flat with a sheet of plastic tucked under on all sides. This way, soil stays evenly moist and the glass or plastic helps maintain an even temperature. Translucent plastic is best because it shades sowings, but clear plastic could be covered with paper.
To make seed sowing almost foolproof, there are seed-starter kits that include everything you need: peat flats, peat pots, sphagnum moss, soil heating cable, labels, marking pencil, plastic bags, even a handbook of advice. I like to use these kits and have had excellent results with them. I soak the sphagnum moss until it absorbs a quantity of water, then squeeze it gently to remove the excess water, but not hard enough to pack it; then rub it gently between my fingers as I fill the flat. Seeds are sprinkled on top of the sphagnum with no covering for fine ones and just a sprinkling of sphagnum for larger ones. The seed flat, slipped into a plastic bag with the end folded over, is set in a somewhat shaded location until germination occurs.