First, we need to open the Steam Serverbrowser.

This can be found under "View", in the top bar, next to "Steam" and "friends".
Under here, you will open "Game Servers"



In this menu, you will open up "Favorites", and click on the Plus in the lower right corner.

In this menu, you will need to enter the servers Public IP, and the Query Port, separated by colon.

If your server is set up correctly, then Steam will detect it. If not, you're gonna have to do some troubleshooting. Check your port forward settings, for instance.

Click OK, and go ahead and close this menu again.

Launch your game, and you will see your server at the top, in the server browser.

Enjoy.