How does one perform istinja' after urinating, what does one do if one forgets it, is istinja' obligatory upon remembering it, and should one change one's undergarments at that time?

"Istinja' is the removal of what exits from the two private parts with water or with three purifying stones or more. If one forgets it and prays, then remembers it, their prayer is valid. It is not obligatory to change underwear; it is sufficient to sprinkle water on it if there is doubt about it being soiled with impurity. However, if one presumes or is certain that it is soiled with impurity, then it must be washed."