How to install a private testnet? I'm looking to make my own NxT testnet for developpement purpose but couldn't find a way to do it.

I'm currently using the normal " testnet " but couldn't find the way to properly configure the nxt-default.propreties so that i could make local testnet. 

Do you guys have any pointers ? or config files ? Will i be able to use my testnet on the same computer or should it be run on a server the same way a public node would ? What about setting up the configure of the genesis block , etc ?

Hope you guys can help
See https://nxtforum.org/testnet/public-testnet-node/ for a description how to setup a testnet node.