You can start by downloading 1.4.6-testnetinabox.tar.xz from here: https://github.com/bcdev-/NXT-testnet-in-a-box.git
Then download the official 1.4.6 [https://bitbucket.org/JeanLucPicard/nxt/src], make a recursive diff and compare changes that I made to create my own testnet.

Then you can create your own genesis block and port changes to 1.7.4.

Happy coding!