Leeks are planted a little differently than onions and garlic because if planted too shallow, they will tend to form more of a bulb, like an onion. In order to force the growth of that desired long white flavorful portion of the leeks, the soil can either be mounded up around the leek as they start to grow, or the starts can be dropped into a hole when transplanted. If simply dropped into a hole, and as the garden is watered and the leeks begin to grow, the soil will slowly fill in around the leek allowing it to properly form. If you are new to growing leeks, here is a YouTube video that explains how to transplant your leek starts into your garden.
I transplanted my leek starts by dropping them into holes that I punched into the soil as I thought this method would allow for more leeks to be planted in a small space. If they appear to be getting so big that they might become be too crowded, I can selectively thin them later in their growing season by harvesting some of the smaller ones in order to create enough space for the others to fully mature.