First of all you'll need the lastest JDK (Java SE Development Kit),
which can be found here:
Java SE Downloads - Sun Developer Network (SDN)
Install it, and reboot your pc.
Now go to
www.moparscape.org TOS, and go to the forums there.
There you can find private servers and clients available for downloading.
You will now need to configure your Router (if your using one),
to redirect all ports to your pc (if im correct).
I can't help you with this since every router is different.
After all of that is done, you need to set up your server (most of the time its a .dos file you need to open)
Then enter the IP of your server (Your IP), and the Port.
Now you and your friends should be able to play on the server.
To modify it, you need to know Java, which is pretty easy when you get used to it.
Hope this helped you.