Which crops you are growing determines how the seeds are sown. Some can be direct sown where they are to grow, others are sown in a seed bed and then transplanted to their final growing position, others need to be sown indoors in pots of compost with heat. Fast maturing crops should be sown little and often to prevent the glut/famine cycle that occurs when long rows are sown in one go. By sowing shorter rows at 10 to 14 day intervals you will harvest just the amount you need over a much longer period.
The vast majority of vegetables are grown by sowing the seed directly into the soil where they are to grow.
To do this you make a shallow trench (called a drill) with a bamboo cane, hand trowel or corner of a Dutch hoe. The seed is sown directly into the drill and covered with soil that is lightly firmed down. Always sow evenly and thinly to avoid excessive thinning out of the crop afterwards. The soil is then watered.
Some crops, especially leafy brassicas, are sown in a seed bed. The resulting young plants are then transferred to their final growing position once they have developed a few true leaves and are large enough to handle. These must be carefully lifted with a hand fork, firmly replanted and then thoroughly watered in.
More on seed sowing outdoors
Protected outdoor sowing and growing
Most seeds won't germinate below a soil temperature of about 7°C (45°F), so early sowings must be delayed until the soil has at least reached this temperature.
One way to speed this up is to warm the soil by putting cloches, clear plastic sheeting or even bottomless plastic bottles over the soil a few weeks before sowing or planting out.
The protection should be kept in place over the seeds and plants for a few weeks or until the weather warms up or the sudden shock of lower temperatures can seriously affect their growth.
To provide plants for earlier crops or for tender crops that can't tolerate early spring temperatures, seed is sown indoors with heat - preferably in a thermostatically-controlled propagator - although germinating on a warm windowsill or in a warm conservatory are other possibilities.
Once the seeds have germinated the seedlings will need to be kept at a cooler temperature, but a cold greenhouse will normally be too cold - especially at night - for the seedlings to grow on without a check to their growth.
The seedlings are then transplanted (pricked out) individually into small 7.5-9cm (3-3.5in) pots or cell trays for growing on.
Before planting outside they will need to be hardened off so that they acclimatise to the temperature. Move the plants to a warm, sheltered position outside on a warm, still day and then bring back in at night. Then leave them outside all day - either in an open cold frame or covered with fleece. This hardening off process should take between 10 and 14 days.
More on seed sowing indoors