What are the legitimate Islamic guidelines for the relationship between an engaged man and woman to avoid falling into forbidden acts?

There is no legitimate relationship between the suitor and the one being proposed to before the marriage contract. She remains a stranger to him, but it is forbidden for anyone else to propose to her if she has accepted the first suitor. He is not permitted to be alone with her or to speak with her except within legitimate Islamic guidelines. It is sunnah to look at her with an exploratory gaze, not one of enjoyment, based on the Prophet's (peace be upon him) sayings: "Have you looked at her?" and "If one of you proposes to a woman, and he is able to look at what would encourage him to marry her, then let him do so."