The 3 obligations on bathing (Ghusl):
1) Gargling: Water must reach right from the lips up to the base of the throat.
2) Inhaling water up to the nose bone: Water must reach through both nostrils and wet the entire area, up to the nose bone.
3) Washing the entire body in such a manner that water flows over every part of the body, not leaving dry any spot equal to the breadth of a hair.
If any food particles are stuck in the teeth, these must be removed. Likewise the nose must be cleaned from any dust stuck inside it, and then the nose washed with water up to the beginning of the nose bone. However, care must be exercised during fasting so as not to overdo it.
Things that make bathing compulsory
1) Discharge of semen in a state of sexual excitement.
2) Ejaculation while asleep.
3) Intercourse, whether semen is discharged or not.
4) Woman completing her menses.
5) Stoppage of impurity for a woman after childbirth (or miscarriage etc.).