Honestly, writing a backstory is incredibly simple when you find a method of developing characters that you're comfortable with and it makes roleplaying a character a far easier affair than some would make it out to be and that you don't need to 'flesh it out' at all. So though I'm really bad at giving people advice, I'm going to try because honestly, this is my first time trying to explain these things to other players and I figure as some people are new to this that I'd try and start sharing my 8 years of tabletop games and D&D experience for others.
As an example I'll use one of my 4.0 characters because she's the most recent I made and still pretty fresh in mind for quick writing.
Start with what race you are, always, the culture generally determines a lot of your character and story to begin with, in this case Dragonborn. Naturally prideful warriors, and have a very 'spartan-esque' culture, parents often teach their children of honour and virtue with skill that also serves to leash a Dragonborn's natural ferocity with military discipline.
Dragonborn is born to her clan's strongest warrior as very much the runt of the litter bringing a fair deal of shame so she's largely ignored by her Mother, but Mommy has no time to waste on a child that will likely end up killed by a stiff breeze so our little heroine uses her Mother's scorn and aforementioned natural ferocity to leave the clan and fuel her personal training, wandering countries determined not to be her mother and to help where she can. In her travels she happens across a small village hiring mercenaries to protect them from soldiers so she offers her own aid, the people of the village feel this is their luck turning as in the center of their village is a small rundown temple to Bahamut, the Platinum Dragon God of justice.
After the battle with the soldiers turns bloody, several of the hired mercenaries being killed with promises of what would be done to the villagers, our Dragonborn forces herself to retreat to rest and heal and finds herself in the little temple where the Priest tends to his canaries and congratulates the dragonborn for her valiant attempt and asks her to rest there, succumbing to injury she loses consciousness and finds herself awake in the very much still intact temple, and village where the old man asks if she'd use her skills as a Paladin of Bahamut, using her heart and skills to help where it's needed to which she eagerly accepts with mild confusion as to why she isn't dead, though the questions are ignored shes taught the skills of a Paladin and sent on her way.
What's beautiful in this backstory is that I didn't write it, I wrote as far as 'and to help where she can.' that fullstop is the end of my writing, and everything after it is what took place in the first campaign I used her in. Starting out as a Dragonborn Barbarian and being trained by the old man as a Dragonborn Paladin. And to explain the rest, the Platinum Dragon often wanders the world in the guise of an old man and his 7 great gold wyrm honour guard disguising themselves as 7 singing canaries, very compassionate to the downtrodden and the weak, and those who choose to fight their own battles in the face adversity. My DM saw this personality about my character and changed the campaign ending to fit, the other mercenaries were other players that had bad luck with the dice rolls and actually died fighting the soldiers, I got a critical success and managed to retreat to the temple on one hitpoint where a God took pity on a Dragonborn cast aside by her own family. This campaign then set the tone for every other campaign I play my dragonborn paladin in and the game itself became her backstory.
Give your character a simple *start* not a backstory, a start. A reason for them to leave their city, village, family, and a reason to be involved in what's happening in the campaign, in this case the Faction. And let the game shape you, rather than you shape it for the game. These often result in some of the most amazing, roleplaying experiences you will encounter as a D&D player, as well as some of the most detailed characters you could hope for. And it could be as simple as something like leaving to find wealth for your family to live in luxury.