What is the legal ruling on designing a website for a postgraduate student for a fee, while providing a detailed explanation of the steps as a training course?

If the student was assigned to complete the website to present it in their name, then what you did is impermissible, and the money earned from it is ill-gotten (khabith). You must purify yourself of it by giving it to the poor and needy or for general charitable causes, because this falls under forbidden deception and reprehensible fraud. It also falls under the Prophet's (peace be upon him) saying: "The one who pretends to have what he has not been given is like one who wears two garments of falsehood." However, if the student does not want the website for this purpose, but rather to benefit from it in a permissible way, then there is no blame upon you for what you did.