بسم الله الرحمن الرحيم
Istinjais to clean whatsoever urine or stool comes out of the front and back ways. If the impurity does not overlap from its source and the part of istinja is cleaned by stone or clay clod so much so that the impurity is gone completely then it is also enough. However it is mansoon and mustahab to do istinja with water after that. And if the impurity spreads equal to the depth of palm i.e. equal to one coin then it is wajib to wash it with water. It is forbidden to do istinja with bone, dung, coal, lead, eatables and costly clothes etc, i.e. istinja should not be done with things which are impure or respectable. It is forbidden to do istinja in standing position. Likewise, turning face or back towards Qiblah while istinja is forbidden. When one goes to toilet he should recite Bismillah before entering the toilet and should read the following dua:
اللهم إنى أعوذ بك من الخبث والخبائث .
One should step in left foot first and while coming out the right foot should be placed out first. After coming out of toilet, the following dua should be read:
غفرانك الحمد الذي أذهب عني الأذى و عافاني .
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.