Plants that have been growing indoors need to be gradually introduced to outdoor conditions, called "hardening off". That means putting them out in semi-shade for a few hours each day, while the outside temps aren't too different from the indoor ones. Gradually increase the amount of outdoor exposure over a period of a month or two, until the plants can take full sun, and night chill. Then plant in the garden.
Actually, Justianne, unless you have an extremely short growing season, leeks should be hardy enough to start directly in the garden.

thankyou

When you do get to plant your seedlings, first use something like a pencil to make a hole deep enough to take them, place your seedling in the hole but don't firm the soil around them. Leave it open and fill with water. The plant will fill the hole very quickly and will put on more width than if they are confined in tight soil!

Great tip! Thanks, Ian.