First you will need to make sure your router is accepting traffic to your desired port (default 25000) and PC. Each Router is set up slightly differently for this but portforward.com has guides for most manufactured routers.

http://portforward.com/english/routers/port_forwarding/

When on this site, you will need to locate your router manufacturer, then the router model. Both of these should be found on the router itself (if any ads pop up, click the x in the top right corner.) The site will then ask you to pick your game, sadly at the time of writing this guide, our game is not listed here, please click on Default Guide which is in the top explanation.

Read this guide through and follow it’s instructions to open the port number for the game.

Once the above has been completed, launch the game and click survive online and then create a server. You can now give yourself and your server a name. Leave the IP as it is and the port should be the same as what you previously set (default 25000). You can also set a password for your server to prevent others from joining. Once you are satisfied with the information click connect.

To connect to a server you will need to have the hosts IP address. They can find this out by typing IP Address into google and it should show your public ip address at the top. Type their IP address into the IP section and then their port number into the port section. Then enter the server password if this has been set. Then click on connect.