What they have to do is to download a ftp-client:
ws-ftp, cute-ftp, filezila are some that comes to mind. In Windows you can also use the explorer by typing ftp://and.the.address.here
To connect they use the username and password you set for the client, and then by typing in the fully qualified domainname (fqdn) for the server or IP-address.
so if your servers fqdn is server.foo.com, then they login to that server with server.foo.com in the address.
Hope this helps.