First, this has grave potential security consequences that you'll need to have your eyes open about.
Do you have a static IP address? If so then if you set your not then you'll need a service such as DynDNS running on some machine on your LAN so your access attempts can be routed to whatever your IP address happens to be at the moment.
(If that paragraph doesn't make sense to you, stop right now.)
Then go to Airport Utility and check mark the "Share disks over WAN" option in the Disks tab. Be sure to set a strong password and manually encrypt anything of value on that disk... or any other disk you have attached, including your Time Machine backups, as all disks will be shared.