Well, the first thing is that you need to have your SQL Server publicly accessible to the world.
I assume you of course understand the security ramifications and will take care of this one way or other.
Anyway, you simply need to know what your SQL Server's public IP is, and open a port in the firewall surrounding your server. Could be the default 1433 but it's good idea to obscure it and use a random port between 1024 to 65535.
Then with the SS authentication, which I assume either you will supply via your code or your user will fill it in just in time, you can construct a connection string. I usually would use DSN-less connection (you can google for Doug J. Steele's VBA sample on this) so I don't have to show the users the default DSN administrator dialogs. If you need the syntax for the connection strings, go to Carl Prothman's site for connection strings.
I hope that will help get you started.