What's the easiest way to set up a private game in Insurgency?

Often I play with friends at a LAN party. We don’t play Insurgency online, so our standard of play is much lower than average, making it completely unenjoyable for us to play on public servers. We’d like to play on a server with nobody else there, but we couldn’t find a button in the GUI to start a LAN server, or anything related.

After some googling, we found some instructions for setting up a LAN server with console commands, but there are several disagreeing sets of instructions, and none of them worked for us.

I also asked the developers in their official livestream, but they just answered, “Just start a custom server,” with no further specifics.

Obviously a LAN server is preferable, because there’s less lag and no need to rely on the one internet connection we’re all sharing, but if it’s easier to set up a private server online, that’s fine. How can we achieve this?