So, you’re going to write a big juicy fic. You’ve watched the show, you’ve fallen in love with the characters, you’ve studied 1973 ;) and you hit the ground running.
Except you suddenly realise that these characters didn’t appear, fully formed, in’73. (Okay, Sam did…but he still has a backstory elsewhere!) There’s a reason they’re world-weary, there’s something that made them happy to solve any dispute with their fists - something made them eager to listen and learn.
And it all lies in their past. A past you know nothing about.
"I tend to live in the past because most of my life is there," said Herb Caen.
We do actually know a little about Gene and Sam, so I’m going to focus my attentions to Ray (big surprise, I know).
So the easiest way to work is to establish a very rough timeline.
He was probably born in the early 1930s - Personally, I go for 1933 (based on the big party we see in the show being his 40th). So the biggest factor of his childhood is going to be that World War 2 broke out when he was about 6.
You can twist this whichever way suits your story - from having Ray live through the full horror of the Manchester Blitz, losing family, friends and missing out on years of proper schooling - to letting him be evacuated to some rich family in the country and living out a life of (relative) luxury, with full schooling and halcyon days.
It’s easy to Google up personal accounts of the War from people who lived in Manchester, or where evacuated, and they’re well worth a read.
How you’ve treated a character during their childhood can then help inform the sort of person they were between growing up and when we first meet them. Think about family, jobs, what could have happened to them to make them into the people we meet in 1973. I’ve always thought of Ray as quite a loner, so I’ve chosen him quite a tragic and lonely past - with an unhealthy dose of violence.
People of Gene and Ray’s age would have done National Service - we know Gene did, as it’s mentioned in the show. But as Ray is only a DS in 1973, I decided he would probably not have joined the police force very early. So instead of having Ray do National Service I thought he may quite possibly have actually joined up. But as Manchester had some thriving docks at the time, and as young Ray would have almost certainly left school at 15, I decided he could join the Navy, not the Army. This opened up a whole world of conflict and fun I could put him through - as the 1950s were pretty busy, in terms of our troops being deployed all over the world.
Once I’ve established my little timeline with a few details, I then write it up and keep it with the fic I’m working on. Here is a timeline for a fic I’ve just finished:
1929 - Gene born
1933 - Ray born
1939 - WWII
1945 - End of WWII
1947 - Chris born
1948 - Gene joined police force
1948 - Ray joins the Navy under fake ID. He is fifteen.
1955 - Gene joined CID (DC)
1956 - Leaves Navy (after Aug 23rd)
1958 - Ray joins police
1960 - Gene promoted to DS
1963 - Ray joins Gene’s team, DC
1965 - Gene promoted to DI
1966 - Chris joins up
1967 - Homosexuality legalized 28th July
1968/69 - promoted to DCI - Ray promoted to DS
1972 - Chris joins CID
It’s only a basic outline, but it gives me a frame to work from.
I’m rather into the whole research thing, so I then expand by finding out exactly which British ships Ray could have served on, and where in the world they would have taken him.
So, basically, backstory can be twisted around to suit your purpose and your story. It’s a lot of fun, if you enjoy a bit of research, and it really helps flesh out your characters, so you can give them memories and thoughts and refer back to their families or past. It also helps define the differences between the characters - Chris was probably a Baby Boom kid, so has never experienced war, but he would have been very aware of the threat of Nuclear War and the 4 minute warnings (he would have been through training for this, as a Police Officer, too).
So my advice would be to establish the major defining factors of your character’s history, then twist them to your own needs. And have fun!