go on your server then tell your friend to check on steam then view menu then server and the last panel show the server where your friend play , tell them to put yours on "favorites" so thats they can find him when you're not online .
First you open your game and run your non-dedicated server; When your server is opened and running nicely you ask then to join your match! make sure everyone has the same mods subscribed on workshop and everyone is up to date with everything! that should cover it!