How was it decided in which order to put the books of the New Testament?

Answer 1

The gospels were placed first, because they describe the beginning of the Christian story. The order was Matthew, Mark, Luke and John, because in those days that was felt to be the order in which they were written. Interestingly, they are in reverse alphabetic order, but that is a coincindence.

Acts of the Apostles followed the gospels because it was a direct continuation of the story.

Paul's epistles are basically in order by size, with the longest first and the shortest last. Hebrews follows because there was uncertainty whether this was also written by Paul. Then come the remaining epistles.

Revelation is a genre of its own, as far as the New Testament is concerned, and is last.