Start Garrys mod and load a map in multiplayer.
In the console do the following.
sv_region 255 (makes your sever show up to everyone)
sv_lan 0
heartbeat
map (Map of choice)
Choosing a map applies the settings above it may take upto 5mins for your server to be updated on the Master Sever List (Sever Browser).
First thing. Your friends will not be able to connect to you through Steam friends as you are connected to the server by an internal IP. You need your external IP.
Go here to view your external IP.
http://www.whatsmyip.org/
Then send this to your friends
"connect <Your IP>"
If you have a password then
"connect <Your IP>; Password <Password>"