How do I route internet through local PPP connection I have on PTY terminal?
pppd automatically registers the PPP network interface upon successful registration!
From my experience performing this on OSX Mavericks 10.9.1, the /etc/ppp directory starts out empty.
Populate /etc/ppp
I had to populate it myself, and I did so from my Ubuntu installation which was already configured properly. These are the files I happened to import based on my needs:

/etc/ppp/options
/etc/ppp/pap-secrets
/etc/chatscripts (optional) - you only import this if you are using one of these chatscripts yourself.

Delete Default Route before running pppd
It might help to remove the default route before running pppd to assist in enforcing that the PPP connection becomes the default connection to the internet:
note: I found this more of an issue on my Ubuntu installation, but haven't had to this on OSX. Still here are the commands:
OSX command: sudo route delete default 
Other Linux:     sudo route del default
When you disconnect pppd and reconnect your regular internet, the route will be restored.