In principle this is a simple as exposing ports 137-137 and/or 445 to the
Internet, and accessing the computer by way of its public IP address.
However, that would be a risky thing to do, as the security on file-sharing
is not particularly good, and these same ports are also vulnerable to a whole
range of exploits, e.g. the DCOM bug.
To achieve this with reasonable security you need to 'tunnel' the connection
through an encrypting protocol such as VPN, SSH or Zebedee. Basically what
these do is to encrypt the traffic and then re-route it through another port
on the same computer. In this case you only expose the encrypted data-port,
not the file-sharing ones. The encryption makes it very much more difficult
to compromise the connection.
It would also be wise to have a fixed IP on your connection, and a firewall
which restricts access to specific source IP addresses.
Hope this helps, if you want to explore it further you need to do some
reading!
