You will need a program called Logmein Hamachi, its very easy to use and has a quick installation and of course you will need Garry's Mod and a friend if you don't have any well then have fun with this guide.

If you are not the host then you must copy and paste their IP (hamachi IP) into console after you put connect like this "connect (Insert Hamachi Ip here)" but first you want the full effect and for this to work, you should both uninstall your addons then think of some new addons and install them then both friends create a collection and subscribe to each others collection and then continue this process and play together doing what ever ♥♥♥♥♥♥ up ♥♥♥♥ they want. narkies..