Princess Diana (my favourite picture of her)
I have several fantasies, and one of them is growing up in the English countryside, preferably in an abbey that I charmingly refer to as “my humble country home” (its real name would probably be something like “The Pomeroy”). I’m Lady Mary Crowley, but like in the ’80s when she would have definitely been a Sloane Ranger. Does that paint an adequate picture? Not yet. My ’80s version of Lady Mary prefers cozy Shetland wool sweaters to Edwardian frocks, Wellington boots to spool heels. She has at least two beautiful sisters, a handsome brother that rides horses, and several Cocker Spaniels that follow her every move. In short, she’s Princess Diana, before and for the short period after she became a princess.

Sloane Ranger Handbook
In 1981, writers Anne Barr and Peter York identified a group of blue-blooded gals in their Sloane Ranger Handbook (the name comes from Sloane Street and Sloane Square, the Rangers’ hangout spot). These twenty-somethings “are upper-class men and women who come ‘up from the country’ to live in London after finishing school or university. Their object is marriage.” That quote is courtesy of a 1984 New York Times article, “On the Trail of London’s Sloane Rangers” — one of the greatest things I’ve ever read. The Sloane Ranger Handbook is very specific. It uses two fictional characters Caroline and Henry as model Sloanes. 
“Caroline will have a mane of thick (but manageable) hair, held back perhaps by a 1960’s velvet-covered hairband. There will be a Hermes scarf somewhere in evidence (perhaps tied around the strap of her shoulder bag). In winter, she will be wearing a Loden coat; in summer a dress (probably Liberty print). The sportif Caroline will wear a lambswool crewneck sweater in a clear colour with the lace collar of a blouse turned over it. She may wear jeans or a skirt in navy, burgundy or olive with coloured pantyhose and low-heeled Gucci shoes.”
Charles and Diana
It’s basically Lady Diana Spencer, the Mama Sloane. Who could forget Diana’s wholesome ensembles, before she became the glamazon princess-in-the-wrong? V-neck sweaters layered over button-up shirts, trousers or “proper” knee length skirts, for lunching at Menage a Trois (her favourite, serving only starters and puddings), and Barbour jackets and Wellies for country getaways with her family. 
Daphne Guinness
While couture-clad Daphne Guinness prefers to shock and awe everyone in her sight in the present day, back in her Sloane Ranger days, she was much more modest. (Other famous Sloanes included Charles’ other mistress Dale Tryon and Jimmy Choo’s former CEO Tamara Mellon.) The Sloanes would never be caught in something so decadent: that would be of poor (and rather unladylike) taste. 

“Sloanes do not believe in high fashion, but they will occasionally allow themselves to be led astray at Joseph. She will shop for dressy dresses at Rodier and Ib Jorgensen. Whole wardrobes are bought at Jaeger. A Loden coat at Gordon Lowes ranges from $182 to $266 depending on whether the removable lining is wool or fur.”
Lady Sarah Spencer (Diana’s sister)
Other than being a proper preppy lady, Sloanes developed their own speak, like “Hohmygod (an exclamation following banal piece of gossip) or “Sweetiedarls, what’s the goss?” (How are you?). Let’s practice, shall we?
Lady Di and Sarah Ferguson
Sarah Ferguson: Sweetiedarls, what’s the goss?
Lady Di: I’m literally dying. I suspect darling Charles is still seeing Camilla.
Sarah Ferguson: Hohmygod!
Lady Di: I know! And the worst thing is that she is not even a top totty (an attractive young woman)!
Sarah Ferguson: Oh Sweetiedarls…
Lady Di: I’m devastated; I think I’ll be off-roading it (in the country) this weekend. Too bad, I really wanted to go to Annabel’s gala. Are you going to attend?
Sarah Ferguson: No, it’s GLO (guest list only).
Lady Di: Oh Sweetiedarls, soon you’ll be a Duchess so you’ll be on all the GLOs. 
Charles and Diana
William and Kate

So excuse me while I retreat to my Abbey, eat Jello with a fork and organize my Hermes scarves.
Further reading: 
“On the Trail of London’s Sloane Rangers,” The New York Times 
“The Sloane Ranger Rides Again,” The Daily Mail (In recent years, Kate Middleton has been credited for reviving the Sloane Ranger style.)