This actually took some figuring out on my own because Tetrinet's help page was very confusing. To put it in simple terms, all you have to do is 1) Go to Server Settings, 2) Click Start Server. That's it. And if you want people to join your server, you give them your IP number (ex. 172.43.75.543). They would join your server by going to Client Settings and putting your IP number in the "server IP" box and pressing "Connect". Both the host and the joiners should go to the "Partyline" tab after pressing Connect or Start Server.
(If you don't know your IP number, you can find out by clicking on "Start" on your Windows taskbar, selecting "Run" from the menu, putting winipcfg in the box, and clicking "Open".)