Augmenting Ghost with new fields for Post and other Models

Hi - looking thru the developer documentation, it seems like it’s not super thorough. I’ve forked Ghost and am hoping to augment the posts with hard-coded schema custom fields. There seem to be a few different locations required to configure and add fields like this (just testing with a simple string field for now).

Would someone be able to walk me through this?

I am also interested in this. Specifically, I’d like to add a “external link” field that contains an arbitrary URL pointing elsewhere on the web. Is there an approved way to extend Ghost like this via a plugin, etc., or will I need to modify the source code of my own installation?