When to plant
If you live in a climate where temperatures don't get far below freezing in the winter, then mid to late summer is the best time to plant leeks. Planting in spring is likely to give a disappointing result. The leeks will mature too quickly and may bolt to seed when the hot dry weather of summer arrives.
If you live in a cooler climate that gets a lot of snow in the winter then planting in spring is probably best. Refer to the guide on the back of the seed packet (if there) or ask at your local garden center.
How to grow leeks
The process of sowing leeks is very similar to sowing onions. One method is to raise the young leeks in seed trays first and then plant out into the garden when they are about 10cm (4 inch) tall. Use a good seed raising mix, scatter some seeds and then sprinkle more seed raising mix on top until the seeds are just covered.
When planting the young seedlings out in the garden, first prepare the soil. A good way to plant the leeks is to use a pencil or straight stick to create the hole, then drop the plant in (roots first of course) and firm the soil gently around the base of the plant. Plant deep to encourage long shanks, but make sure there is still plenty of top poking out of the soil.
Alternatively you can plant leek seeds directly into the garden. Prepare the soil, create a small trench (less than 1cm or 1/2 inch deep), scatter the seeds and cover. Scatter more seed than you think you will need, then when plants are large enough thin out to about 12cm (5 inch) apart.
As the plants grow, mound soil up around the stems as in the picture. This encourages long shanks (the shank is the white or light green part which you use in cooking).
After 2-3 months the leeks will start to fatten. You can use at any stage once they are fat enough to be worth picking. Leaving leeks in the ground during winter is usually fine, but use before the following spring or they will probably go to seed.