Sandy Powell could make a pair of hot pants in pink corduroy for her doll before she could make her bed. By 13, she was sewing exotic flaps of purple paisley into her straight leg jeans.

Now 45, she is an Oscar-winning costume designer, dressing Cate Blanchett and Kate Beckinsale in frocks of rare magnificence for Martin Scorsese’s new film, “The Aviator.”

“This is my favorite,” she says, holding up an evening dress for Ms. Blanchett apparently crafted from liquid mustard, a rayon waterfall of drapes and folds cinched with a deep waistband of gold studs.

Ms. Powell may be famous for her clothes, but as a costume designer she slaves in the shadows and plays down her role: “A costume designer’s contribution is to help make some believable characters, that’s all.”

And yet she does much more than this; she is the conduit for the director’s vision. Which is why Mr. Scorsese chose Ms. Powell and entrusted her with a budget of $2 million for the costumes on “The Aviator.” (She has a much lower budget on her latest film, “Mrs. Henderson Presents,” directed by Stephen Frears.)

Chronicling the early life of Howard Hughes, the eccentric American billionaire, “The Aviator” covers the period from the 1920s to the 1940s when Hughes (Leonardo DiCaprio) was making big Hollywood films, flying and designing fast planes,and romancing the screen legends Katharine Hepburn (Ms. Blanchett) and Ava Gardner (Ms. Beckinsale).

All of this means that Ms. Powell, a costume veteran of 30 films, is in her element. You only have to see Gwyneth Paltrow’s eau-de-nil and gold dressing gown in “Shakespeare in Love” (for which Ms. Powell won her Oscar) or Daniel Day Lewis’s intricately patterned waistcoats in “Gangs of New York” to see that Ms. Powell is emerging as the Kandinsky of costume design.

“Color is the most important thing,” Ms. Powell asserts. She even directs her costume-dyer, John Cowell, to tint shirts to match eye color. When she asks for “Aidan-Quinn-Eye Blue,” Mr. Cowell knows exactly the shade she means. Ms. Powell often uses color to unlock her characters. In “The Aviator,” she made use of strong autumnal brown, apricot, rust, and copper for Ms. Blanchett to underline Hepburn’s languor and outdoorsiness; and red, green, turquoise, and peacock-blue for Ms. Beckinsale to capture the volatile, violent relationship Gardner had with Hughes.

On a typical film, Ms. Powell might head a department of 10 to 20 people. There were far more on “The Aviator.” It was shot in Los Angeles and Montréal, and Ms. Powell had a team in each city, many of them doing nothing but making costumes. She also had a shopper in New York.

She has her assistant, Debbie Scott, with her at all times. Her aides, Mr. Cowell and Annie Hadley, a pattern cutter, have worked with her on every film for the past 10 years. On set, she has a wardrobe supervisor in charge of dressing extras. We meet at Shepperton Studios, in Middlesex, England, where Ms. Powell is wearing a loud mix of patterns that slap you awake on this gray, late-autumn morning.

This is Ms. Powell’s signature – knowing not only which colors suit which skin tones, but also how colors look through a lens. This was made more complicated on “The Aviator” by Mr. Scorsese’s recreating the techniques of early color filmmaking. The scenes set in the 1920s and 1930s are digitally enhanced to recreate two-strip Technicolor; later scenes in three strip. The two-strip color process makes scenes look like old hand-tinted postcards.

“There is no blue, but lots of pinks and greens,” Ms. Powell explains. “If a guy is wearing a blue suit it would come out green. Three-strip Technicolor is fullon, heightened color; the blues are really blue, the reds are really red – pure color. I knew more where I was with that, but with earlier scenes, it was really difficult working out how colors would react.”

To highlight the airbrushed Hollywood glamour of the age, Scorsese achieves a glossy, highly polished look in “The Aviator,” and Ms. Powell’s costumes play their part beautifully: spangled showgirls, flowing gowns, fur wraps, and knife-edge pleats.

“‘The Aviator’ is about creating a period and a feel, a slice of somebody’s life,” Ms. Powell says. It is historically accurate, but this isn’t always the case in films she works on.

“In ‘Gangs of New York,’ set in the 1860s, we were creating a world which was not exactly imaginary, but we exaggerated things. We made the shirts much longer, for example. We had a lot more artistic license. Here, I wouldn’t want actors walking into a scene and people thinking, ‘My God, what are they wearing?'”

To this end, she had to persuade Ms. Blanchett that her trouser crotch was meant to hang by her knees.

“Actors expect to have it up high, tight bum and everything,” she observes.

Similarly Mr. DiCaprio had to get used to his high-waisted trousers.

“In the 1940s, waists are way above the navel and, of course, the actors all want to pull the trousers down and hang them off their hips,” Ms. Powell says. “People are not comfortable having a waist anymore.” She even gave Mr. Di-Caprio a corset. “Forties jackets are really tight and I wanted that really nipped-in, poker-up-the-back look.”

But Ms. Powell’s biggest problem is our obsession with exercise.

“Muscled bodies aren’t right for this period. You don’t want to see rippling, sinewy arms emerging from an evening dress,” she says. “You want to see flesh, and Kate was perfect. She has a fantastic, voluptuous figure and I begged her not to lose weight.”

Getting to dress some of the most beautiful bodies in the world is arguably Ms. Powell’s biggest perk. She gets in close.

“Underwear is the most important thing,” she explains. “It provides the silhouette for everything that goes on top. “Scenes set in the 1920s and 1930s are underpinned with soft bras and French knickers, with actresses progressing towards a more structured “lift and separate” look in the 1940s.

Mr. DiCaprio wears boxer shorts and vests in the film. “Everybody still thinks of him as Jack in ‘Titanic,’ like a boy,” she smiles. “But he is a big man, surprisingly. About 6 foot.”

Ms. Powell uses a mixture of her own designs, period clothes bought from vintage shops, markets, and fashion fairs, and outfits from big rental houses.

“It’s really difficult to get hold of old fabrics now, so I sometimes have to use new fabrics, but I can always tell the difference. They don’t have the same weight, they don’t hang, and you can’t do the same cut with those clothes.”

Ms. Blanchett’s mustard dress is based on a very old decrepit dress rented from a rental house. Ms. Powell redesigned it using original 1940s rayon found in a fabric store in Los Angeles that kept old stock.

“There is obviously more vintage stuff in America. It’s a bigger country. There are even people who collect and sell and deal vintage underwear, which was useful to use or copy. The girdles and all that stuff you can still buy. Big old American ladies wear them.”

Period shoes present another problem.

“Even if you do find shoes that haven’t been worn, the chances are they’re not going to fit a modern foot. We have much bigger feet now, so I have them made by a company in Italy. Men’s shoes are much easier. Classic shoes haven’t changed that much. I put Leo into shoes by Church’s.”

The real test of stamina for a costume designer is not the filming itself, of course, but the months preceding it, which are spent researching, shopping, meeting actors, and designing. Ms. Powell worked on “The Aviator” for seven months. Four months were spent in pre-production, poring over archival material from the period including Sears catalogues, plus films and books sent by Mr. Scorsese’s office.

Ms. Powell doesn’t come up with specifics until she has met the actors. At breakneck speed, hemlines are pinned, silhouettes discussed, designs drawn, colors matched, and accessories added. Once her team has made the costumes, they have to be aged: suits are dampened and tumble-dried; pockets are loaded with weights; grease marks are sprayed on – so are contours to emphasize waists.

“I certainly do that for all leading actresses,” Mr. Cowell says. “I work dye into their dresses to flatter them as much as possible.”

“Basically, my job is to get the clothes on the actors,” Ms. Powell explains. “And then to make sure that the first time they wear them on set, everything is how it should be.”

“What I feel like doing next is a big costume number from the Renaissance,” she says. “It’s a sumptuous period, isn’t it? Nice to look at, nice to research.”

It is a period that she would no doubt do proud.