Let me preface this by saying that much of this post may be considered subjective. It's not meant to tell people what
to write, or how to write, as both of those are things that really need to come from the writer themselves, but hopefully this can serve as inspiration on some level.
Before you start making a character, chances are you'll already have some sort of core concept in mind for them. This may not be the case if it's
your first character on the game, but it's an important first step for writing a history. For example, I started out with Erikson as 'a strong
half-demon with a penchant for wrestling.' Fairly generic, but it's enough that further details can grow from there.
Once you've got a core concept, you should consider how your character developed into that concept.
For characters with demonic aspects this may tend to be a bit easier, since you can develop the character by developing their race. If it's not one
from the show, you could try and write a culture for them. What's their home dimension like, if not Earth? What's their society like? From
there, you can figure out how your character fit in with that culture, if they even knew about it.
For human characters, this may seem a bit more mundane, but it doesn't have to be. You'll probably want to start with where they're from and
who their family is or was. This may not be a key point for a character, but it's good to at least have it in mind in case it comes up. Chances
are you're already a demon hunter or serial killer by the time you reach Sunnydale. Why is that? Why don't the expectancy charms work on
you? By all means, feel free to not have this be the case, but assuming it is, you'll want to figure out why. Your class will likely suggest some
possibilities, though going against the mould to some degree will often result in a more unique feel for your character.
Vampires have some universal traits that should provide some definition for your character. For instance, who sired you? What effect
did your siring have on your character's psyche? What happened to your sire? Other than questions about your character as a vampire,
you should figure out who they were as a human. What era did they live in? Are they a fledgling, or have they been around for a while?
If they've lived for long, you'll probably need a longer history on their exploits as a vampire.
Once you've got some ideas going on, you'll probably want to figure out what format you want to write the background in. Consider the
reader. How are they finding out the information you're giving them? Usually having a high research skill either means having skill in
computers or in book research. If it was or Giles researching your character, what would they find? If you're an ancient being
of evil or an infamous vampire, your history's likely well-documented and standard story format would fit. If an organisation would have
a file on you, you might consider writing in the format of a dossier that a character might find.
In a similar vein, I don't think that you should necessarily feel obligated to fill in every single detail about your character's history, but at
the same time, keep in mind that whoever's researching you did put in a lot of experience just to be able to do that. A good history should
provide an RP hook without substituting for actual RP. Give too much information and you do the latter, give too little and you fail to
do the former.