Here is a site with instructions – I followed some very similar, only needed to set up something like Filezilla afterwards, it was that quick & simple!
FTP Site Setup (IIS 6.0)
When you say “non LAN clients”, you mean the internet ?
If so, the computer will need to be accesible from the internet (i.e. have a public ip address, or be mapped to one).
You could find these articles useful also:
The Windows FileZilla server application can be found here. You will need to open a port on the firewall to permit inbound connections on ports 20 & 21 for FTP. You will