LONDON (AP) _ In the attic of her home, Lindka Cierach is putting the finishing touches on princess-to-be Sarah Ferguson's wedding gown - a dress that will be seen by millions around the globe and copied in a department store for sale five hours after the royal wedding.

The gown is one of the few remaining secrets of Wednesday's extravaganza, when Miss Ferguson and Prince Andrew are to marry. Miss Cierach has consulted a security expert and put special one-way screens on her attic design studio windows to see that it remains so.

Though she was little known in London's fashion circles before Miss Ferguson chose her, Miss Cierach was tipped as ''the hottest society dressmaker'' earlier this year by the high-society magazine Tatler.

Working from her modest house in London's blue-collar Fulham district, the 34-year-old designer had quietly built up a clientele that includes the Duchess of Kent, Queen Anne-Marie of Greece and Miss Ferguson's apartment-mate Carolyn Beckwith-Smith, who was married July 5.

Her wedding gowns, which cost at least $7,500, are known for their exquisite fabrics and beautiful detail. She lets her clients decide the period, mood and look they want.

Lady Rose Cecil, daughter of the Marquis of Salisbury, had mink-trimmed sleeves and bodice. Socialite Charlotte Monckton had an 18-foot train.

Jet-setter Pandora Stevens had 60,000 hand-sewn pearls, crystals and sequins. Socialite Cynthia Menzies, following a ''Gone With The Wind'' theme, had an embroidered and beaded skirt with eight tiers over a crinoline.

''A wedding day is very special for a girl,'' Miss Cierach was quoted as saying by the Daily Express. ''Because the whole day is so important, they have to feel total confidence in what they are wearing. So I never push hard with what I think they should look like.''

Several British papers have predicted Miss Ferguson's dress will be Edwardian in style with a bustle. If she follows royal tradition, Miss Ferguson will choose silk from the Lullingstone Silk Farm, 120 miles southwest of London, like Queen Elizabeth II and Princess Diana.

Miss Cierach has refused to discuss anything about the dress but she was quoted as telling The Sunday Times of her most famous customer: ''Fergie is a most delightful person with a terrific sense of humor. She is smashing to work with and could not be nicer.''

Born in Lesotho in southern Africa, Miss Cierach spent her early childhood in Uganda with her English mother and Polish-born father, a World War II hero who traveled eastern and southern Africa doing survey work for maps.

She came to England when she was 8 to go to school. She was spurred to become a designer by her boss at Vogue Magazine, where she worked as a secretary. She was a star student at the London College of Fashion, spent six months working for Japanese designer Yuki and then started her own business in 1978.

Describing her design philosophy, she said: ''I think it is vital to be able to see what will really suit a personality to make the very most of a woman. I want them to feel as I do when I am at my best - confident, sensuous, fun and attractive.''

Whatever style Miss Cierach comes up with for Miss Ferguson, garment manufacturer Sidney Ellis said this summer's brides will be able to buy an exact copy five hours later in one of London's major department stores for $1,275.

Ellis said he did the same fast copying at the 1981 wedding of Prince Charles and Lady Diana Spencer and previously at the weddings of Princess Anne and Princess Margaret.

Prince Andrew said Miss Ferguson didn't want him to know anything about her wedding gown.

''I will not see the dress until it comes up the aisle,'' he said.

But Andrew said he'll have one secret for the bride himself: ''She doesn't know anything about the honeymoon.''