Quote from Gutholz :

The PC who is hosting:
Multiplayer - Start new host
IP Adress: auto
Visibility: local
click GO

The PC who is joining:
Multiplayer - Join specific host
Join host on: local network
Host IP adress: enter the local IP of host PC (begins with 192.168...)

Is this using hamachi? What If me and my friend aren't connected to the same wifi/router? Then we just need to make a hamachi server?