Do not sow the seed too early in the year or the plants will be ready for planting out too soon, before the danger of frost is past. Late February and March is quite soon enough — the later sown ones eventually catch the others up because of having more daylight and natural warmth.
Nearly fill seed trays or small pots with a good moist seed-sowing compost and sprinkle the seed thinly on top, then cover lightly with more compost. If the compost was damp enough in the first place, it should not need watering again now, otherwise damp the surface with a watering can using a very fine rose, or a trigger-spray of tepid water. This is a tricky job because if you are not careful you will wash all the seed out. Cover the trays with a piece of clear plastic, glass, or cling film and a sheet of newspaper, or the plastic dome of the propagator and keep at a constant temperature of no lower than 65°F (18°C), either in the house, greenhouse, or heated propagator. Germination time varies according to the type of plant being grown. Remove the glass and newspaper as soon as the seed germinates (you can leave the propagator lid on with the vents open). When the seedlings are large enough to be handled, prick them out by lifting them gently from the compost with the end of a knife or similar small tool, and replant in trays of potting compost about 11/2 in. (40 mm) apart. Only handle seedlings by the leaves, never the stems, which may be crushed so they rot off at the base. You can also pot up the baby plants into individual containers. This makes it easier to plant them out without damaging the mass of fibrous roots they make.
Grow the plants on in the warmth in these trays or pots, and a week or two before they are due to be planted out in the garden, harden them off. This means that they are gradually introduced to the cooler conditions they will meet when they are outside. If you have an opening cold frame, place them uncovered in this during the day, but cover them over at night to avoid damage by frost. Otherwise, place outside during the daytime and bring into a cool room, shed or garage for the night.
Treat starter kits similarly — follow the instructions, which vary according to the producer, until the plants are ready for pricking out, then follow the above procedure.
Seedlings bought from the seed producers are usually despatched to the purchaser at the right time for potting on into bigger pots.
If you have nowhere to raise half-hardy annuals indoors, many of them will germinate in their outdoor flowering positions if sown like hardy annuals in May. They will be later coming into flower, but you may feel it is worth a try.