Firstly: Defining Istinja’
Istinja’ linguistically: is the morphological state of istif`al from the root word najw, meaning to cut; so it is as if one has ‘cut off’ filth from himself by performing istinja’.
It has also been said that the root word whence istinja’ comes is najwah, which is a plateau or upland; so it is as if when someone needs to relieve themselves, they seek out some highland to cover themselves, so it was said to one who does this (in the singular masculine present verb form): istanja.
Istinja’ technically: removing filth from the exit, using water or stones.  Al-Nawawi said: “Istitabah, istinja’, and istijmar are all methods of cleaning after relieving oneself. Istitabah and istinja’ are by using water and stones, whereas istijmar is solely by using stones, derived from jimar, which are small stones.” Tahrir Alfaz al-Tanbih, p. 36; also see al-Sharh al-Mumti` by Ibn `Uthaymin (122/1).
Secondly: The Ruling of Istinja’
Istinja’ is wajib  Wajib is one of the five Islamic legal rulings, and it means that the matter in question is a must as well as rewarding if committed, so legally obligatory. if its reason is there, which is a najas substance exiting from either the front or back passages. This is the position of the majority: the most common opinion among the Malikis, Shafi`is, and Hanbalis.
Thirdly: The Ruling of Relying on Another for Istinja’ due to Inability
Whoever is physically unable to perform istinja’ himself, then another who can legally look at his `awrah  `Awrah refers to a place of embarrassment and/or weakness, and usually refers to a person’s private parts. However, technically in a legal context, it is not exclusively so, and differs depending on the context and the parties involved (e.g. a believing woman’s `awrah in front of a believing man is different from a believing woman, different once again in front of a disbelieving woman [according to the majority], etc.) must help him.  In the Permanent Committee’s verdict: “It is permissible for a female nurse to view a man’s `awrah if there be a need and there is no man available to do so. Allah exalted says: ‘Be mindful of Allah as much as you can.’ If a man is able and available to perform this task then it is haram for female nurses to do so.” The Verdicts of the Permanent Committee – The First Collection (344/26). The majority have explicitly mentioned this from among: the Hanafis, Malikis, and Hanbalis.
Fourthly: The Ruling of the Intention for Istinja’
The intention is not one of the conditions of istinja’, and this is by agreement of the four schools of jurisprudence.
Fifthly: The Place of Istinja’ The Anus
The anus and whatever parts of the skin around it have been affected with najas must be washed, and this is by agreement of the four schools of jurisprudence. The Penis
The place where the urine exits – the external hole of the urethra – as well as any other part it comes in contact with must be washed. It is not necessary to wash the head or shaft of the penis as long as urine has not been in contact with them. This is by agreement of the four schools of jurisprudence.