Ash's guide hurts me deep inside.
Deep.
Deep inside.
It hurts me further that it is so heavily referenced.
That said,
overall Ash's guide is not bad, because it puts one on a train of thought, the problem is that Ash's guide causes one to tend to
overdo it. Which I will kinda get to later.
It's pretty simple, imo. Ask yourself the following questions, both of which are pretty basic, and neither need incredible detail:
Where did my character come from? (Homeworld, upbringing, etc)
What is my character's motivation? (What does he/she want to do, and what drove them to this)
Be as open as possible while still answering the questions. This is very important. I strongly stress
against overly detailed backgrounds, because when that happens you fall into a trap, specifically your character won't
develop much. There is no leeway. (<-- the aforementioned problem with following Ash's guide, imo)
The other thing that this helps is you can write other people into gaps in your background, and if a certain character isn't interesting enough, you can adjust them on the fly because you have a large, blank slate to fill in later.
Round them out, too. If it's a good guy, give them negative qualities, if it's a bad guy, give them some heavily humanizing qualities. Three dimensional 4tw.