How to setup real SFTP in Yosemite?

Question

There's lots and lots of information online about how to setup remote access to my mac using SSH and SFTP in Yosemite...but apparently none of it is actually useful for genuinely  remote access...access from outside my network, access from miles away.  Because here is what I have learned:
The ip address listed in the Sharing pane when you access remote login is internal.  it's some version of 192.168.x.x, which is essentially everybody's computer and utterly useless from outside the network.
So my actual public ip as assigned by my service provider is really my IP if you are trying to get to my computer from outside.
But that IP doesn't "just work" via SSH or, much more importantly for me, SFTP.  Evidently that involves something called "port forwarding" which one used to be able to do via Airport Utility, but is no longer possible because such options do not exist in the current version of the utility.
(What I want to achieve is for a friend of mine, a friend on a mac about 4 miles away,  to be able to directly access very large files on my computer. I don't want to use clouds or public sites or anything like that, I just want to be my own private little FTP server just for her. That used to be no big deal.)
So... is there a solution for this?

Answer 1

Dropbox may be a better option if you can both have accounts with enough storage, it is a simple setup process that can be forgotten about.
Bittorrent sync is another option that mirrors data between several endpoints. You should get better performance if the files are not being transferred over the network (Dropbox & Bittorrent sync create copies at each end).
ssh & port forwarding allows you to use sftp, if you do some more setup you can also use sshfuse to mount the remote content as a network disk, it's a plugin for http://osxfuse.github.io
Just be aware that many scripts & botnets are constantly attempting to login on visible ssh ports on routers across the internet. Enabling ssh to outside access means that any user on you Mac can be a potential login - have good passwords set on ALL user accounts, and do not enable the root account password. There are also options to disable passwords in ssh (& use ssh keys only) if you want better security.
ssh is good, but it is not trivial to setup and it won't open ports for you. Apps like bittorrent sync can use UPnP to request port forwarding rules, to make setup easier.
Back to My Mac is OK, provided you are willing to hand over the keys to your Apple ID.
P.S. You keep mentioning ftp, please avoid ftp. ftp can use plain text passwords that can be sniffed over the network, avoid opening the network to inbound ftp traffic too.