Start with how old you want your character to be. Thats one of the easier ones. It will eventually lead up to life events , but we will get to that.
Now , everybody (except if they are vat grown) will have a father and a mother. Focus on them. What were they like. What did they do. Are they still alive ? If not , why ? Is your character still in contact ? ....
Keep asking why. Make yourself feel like a 5 year old that keeps asking why. Why. Why (Also known as the Why*3 method).
Add family members if you want to. What were they like ? Are they still alive ?
Next up , everybody was born somewhere. Pick a cool spot. Take a look at the map we have (I think its America only) or read up on the timeline on the area. Pick a cool city that might have survived or make a general location ( Somewhere east-anywhere, a nameless dump near germany). Flesh it out a little , based on your character. If you know a second language , it will help give that gravitas. If not , don't sweat the small stuff , chances of you comming from the same city as the next guy are slim and it probably was a big city.
Now we get to the harder part , but you should already have a pretty decent amount of text.
Life events ; Married ? Childeren ? Horrible bedtime stories ? Killed a guy before ? What did he do to get the skills he has now ? All the life events you could expect for someone of that age and more. Keep asking those Why's. Why, why why. The why questions are really important to get to the bottom of a motivation.
Example ; you killed a guy.
Why ? In self defense.
Why ? The guy was going to rob him and kill him and his friends.
Boom , pretty cool backstory and you can now tie emotions to it (like guilt or becoming less human and more cruel). And its a lot more interesting than writing that you killed a guy and moving on.
Then what is left is the reason why your character came to the city. Nobody just picks up their stuff one day and tries to come to a city without any reasons.
Maybe the character heard of the riches. Of the gangs. maybe the character is on the run. Maybe he is looking for someone. Maybe someone is looking for him.
Keep asking those why's. Why why why.
And if you finish that , you can flesh it out , but you won't need much more to have a good history. Just keep asking yourself why.