While it is technically possible to edit d4 personalities with any text editor, there is no need to do this: there ia a program tailored exactly for this job: the Personality Builder. It comes included with the Titan PC suite, is installed on every console (you'll love to have mouse and keyboard though), and is also available as separate download. Also there is a Quick Start Guide available.
There are several videos available about the personality builder - even in various languages: simply search on youtube.
At the moment, the most detailled videos are made by The Bright Pixel:
And here is something about various aspects of building a personality: