Is the Istikhara prayer recommended for a situation where one's heart is attached to a girl who refused marriage because the suitor is still a student, and what is the method of performing it, and should its supplication be made during the prayer or after it, and how are its results seen?

You must be content with Allah's decree. There is no harm in proposing to the girl again. At that point, the Istikhara prayer becomes permissible. However, if you have no intention of proposing, it is not permissible.

There is no specific way to know the outcome of Istikhara. Rather, it is about proceeding with what your soul finds comfort in, then accepting and submitting to Allah's decree, and knowing that what Allah has chosen is the best.

You must strive to stop thinking about this girl and occupy yourself with what benefits you in your religious and worldly affairs.