Ghusal becomes fard (obligation) on man either because of wet dream and ejaculation, or because of intercourse with wife. On women it becomes fard due to menstruation or because of intercourse with husband.
Advocate Faiz Syed said that the method of Ghusal is very easy and can be summarized by saying that it is washing the whole body without any part of body being dry.
For ghusal you should first clean your private parts, then you should perform wadhu (wazu) and then wash your head so that water reaches the scalp and hairs become wet. Then according to Sunnah wash your right side body parts and then body parts of left side.
Water should reach every part of the body three times and it is also sunnah not to use towel after ghusal.