Non-dedicated servers are straightforward. Pick Servers->host server in the Arma launcher or Multiplayer->Server Browser in game. Change LAN to internet in the dropdown. Either check the UPNP box or forward port 2302 on your router. Steam friends should then be able to join your server once you've started Antistasi.

Everything else is much like running singleplayer, assuming that you've got that working already. You may want to change the automatic commander selection and server membership options in game parameters (on the screen where you pick which squad you're in).