What is the method for performing the major ablution (ghusl), and must it be done after intercourse during Ramadan before Fajr or in the morning before noon?

There is no impediment to delaying the ghusl (ritual bath) after intercourse at night until the rising of dawn. The fast of the one who does so is valid, and there is no need for him to make up the fast, based on the hadith narrated by Aisha and Umm Salamah (may Allah be pleased with them) that the Prophet (peace be upon him) would wake up in a state of janabah (ritual impurity requiring ghusl) after intercourse, then perform ghusl and fast. It is, however, recommended to hasten the ghusl before dawn so that one can perform the Fajr prayer at the beginning of its time and with the congregation. As for delaying the ghusl until midday, this is not permissible, because it leads to missing the Fajr prayer.