How is istinja' (cleansing oneself after relieving oneself) performed after urinating, and what should a person do if they forget to perform istinja' and remember it later, and in this case, is it obligatory for them to change their undergarments?

The majority of scholars hold the view that it is obligatory to perform istinja' after urinating and defecating, either with water or with a stone or what takes its place. If one forgets to do so, it becomes obligatory to perform it. If any impurity attaches to one's clothes or one notices wetness, it is obligatory to wash it. The method of istinja' is to gently stroke the penis from its base to its tip, then shake it three times gently before performing istinja'.