How do you create user personas

I'm currently reading a book about how to build a good following with marketing your business/building a brand and one of the steps it advises is to create several customer personas. As described, this is very similar to making user personas as part of a UI/UX design, so I'm a bit familiar with it. But the thing that has my brain stalling is, how do you fill in the details or is it normal to pull details out of your bum (aka guessing)? For example after you write down the bigger picture items like gender, name and other basic demographics, you should fill out details like political affiliation, hobbies, etc.

More specifically, how do I do this without making them a complete fiction which isn't going to help me at all?

Also, is this something that can start as near-complete fiction but gets refined and replaced with facts over time as I learn about my customers?