How to enable invasion mode?

I want to fight the monsters!

Try this way:
Launch Xonotic and select “Single Player” from the main menu.
Select “New Game” and choose the map you want to play.
Once the map has loaded, open the console by pressing the tilde key (~).
Type “sv_gametype invasion” (without quotes) into the console and press Enter.
Close the console by pressing the tilde key (~) again.
Start the game and enjoy fighting the monsters!

What did you do?