Answer
In the name of Allah, the obligatory washings are the ones of Janabat, menstruation, puerperium, the dead, touching the dead, and Istihadha [pre/post menstruation] in case they are medium or heavy as per the details mentioned in the Guide of Islamic Rules.
As for the way of washing, if you have a lot of water in front of you, you can wash a full body, by throwing your body in it after the intention, and here the water must reach all of your body at one time. Other than this way (Irtimas), after removing the impurity you can wash the head and neck, then the left part and then the right part. It is recommended to wash the neck again when washing the two parts. The water must reach to all the parts of the body for sure. For more details, consult our Guide of Islamic Rules. Allah knows best.