Cut the leeks into the size and shape that you want. You can only clean the leeks effectively after cutting them as the dirt is hiding in between all those layers. In general, halve them lengthwise, then sliced crosswise into 1/4-inch thick pieces OR you can just slice them into rings (as shown in the photo above)
If you cut them into rings, use your fingers to push the layers out
Soak in water:
Transfer the leek pieces to a large bowl of cold water and stir to loosen the dirt. Make sure you stir it!
Let stand for 5 minutes to let dirt and sand settle to the bottom
Remove leeks from soaking water:
Gently lift leeks out of the water with your hands to a colander, try not to stir up any dirt that has settled to the bottom. Do not simply pour the dirty water out from the bowls with the leeks in there. If you do so, the dirt will get caught in the leeks again
Discard the dirty water and rinse the bowl clean and repeat this process again from soaking the leeks, probably another 2 times until you no longer see any dirt in the water
Ready to be used:
If you don't see any more dirt or sand in the water, the leeks are ready to be used in your cooking