Trim ⅛ inch off the root ends, then remove the green tops and any tough, damaged outer fronds and save for stock. Cut the leeks in half lengthwise and then into quarters. For matchsticks, continue to slice lengthwise. For dice, cut the quarters crosswise. Each square separates easily.
Leeks can be really sandy, so it’s important to clean them well. Fill a bowl with cool water, add the diced leeks, stir them around, then lift them out with your hands or a slotted spoon into another bowl of water and repeat the process. Lift and drain in a colander. This is the way I wash salad greens as well.