Chop up your lemon grass, shallots, garlic, and coriander stalks. Sprinkle with a small dash of salt to help release liquid and keep chopping away until you've got a nice little paste.
Mix with the oil, dried coriander, sugar, fish sauce, and another small touch of salt - fish sauce is extremely salty on its own so don't be liberal with the amount of salt you add. Pound and mix until you've got a nice, mushy mixture
Coat and marinade the chicken for at least 20-30 minutes. Longer the better.
Grill for 3-5 minutes per side, depending on how thick your piece of chicken is.