Two widely used versions of Tom Yum are present in Thailand. One is cooked with coconut milk and the other one, a lighter version, uses water. Depending on what you'd like to use, the procedure is quiet the same.
In a hot wok, bring some coconut milk or water to a boil then simmer with galangal and lemon grass. Stir every so often so that the flavors infuse well. Add prawns, straw mushrooms, thick coconut cream, tom yum paste, and lime leaves. If you want this to be extra hot and spicy, add Thai chillies. Continue to simmer until the straw mushrooms are tender.
Add fish sauce and lime juice to taste. Garnish with coriander leaves.