Correct way to backup via Time Machine to a remote Time Capsule
You would want to run a VPN server on the network where the Time Capsule sits so that your remote machine can log in over the internet as if it were a local computer.

This pokes holes in the network since most people don't allow their Time Capsule to be reachable by anyone on the internet and have their entire home network hidden behind a router with a local and non-routable internet address.

OS X server would be an inexpensive and well documented / supported manner of setting up a VPN or sharing any hard drive with others for Time Machine backups as well as enabling a local Time Capsule to be reachable directly by VPN clients.