I knew the dress that transforms Cinderella into a beautiful princess needed to be big – very big. It had to be a showstopper.

Sadly, there was no real-life fairy godmother to conjure it up. In fact, it took a team of 20 people 4,000 hours to create eight versions of this very special gown.

The result? Rave reviews when the film, directed by Kenneth Branagh, opened in the UK a week ago.

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Rave reviews: Cinderella is played by Downton Abbey’s Lily James. The film has had excellent reviews with Vanity Fair calling it ‘genuinely magical’

Vanity Fair called it ‘genuinely magical’, while fans have said the dress is one of the most spectacular costumes ever.

But how do you go about creating the ultimate fairytale gown – and making it look truly magical on screen?

Cinderella – played by Downton Abbey’s Lily James – has to dance and run away into the night, and I wanted the dress to look as light as air, a watercolour in motion.

Costume-maker Jane Law and I achieved that with the choice of fabric and by using many different shades of shimmering blue.

Behind the many layers, the gown is a feat of structural engineering. We started with the underwear: the corset and the crinoline (skirt cage), which was made of steel

Fairytale: Cinderella - Lily James - captivates her prince Richard Madden at the ball

AND THE WAIST? IT'S AN ILLUSION! • Creator Sandy Powell says outrage over Lily’s slender waist was misplaced. It’s actually an optical illusion – her waist looks tiny because the skirt is so huge. • The top layer of the gown is silk crepeline, a very lightweight, fine silk. The layers underneath are made up of a synthetic called yumissima, an incredibly light (and very expensive, about £150 a metre) material which floats when thrown in the air. • Lily always went to the loo before being stitched into the outfit... but if nature called again, a mini camping portable loo was slipped under her dress. • The designer and her team are sworn to silence about the cost, although some estimates have put it at about £11,000 per dress. But you can buy a similar design on the High Street for £100 – about 100 times less. Advertisement

Behind the many layers, the gown is a feat of structural engineering. We started with the underwear: the corset and the crinoline (skirt cage), which was made of steel.

Each of the eight versions of the gown is also slightly different. One was 2in shorter and a couple were 4in for the times when Lily had to run; another had holes cut in the sides of the skirt for harness work.

The hems got incredibly dirty as the shoot went on. That proved a headache – these dresses aren’t something you can just throw in the wash.

And it also wasn’t the most practical design. During filming, Lily and Helena Bonham Carter, who plays her fairy godmother and wears an equally huge gown, had to get used to it taking an age to be dressed. In the end, we got it down to a 20 minutes.

THAT DRESS... BY NUMBERS 3 Miles of thread in the hems 250 Metres of fabric in each dress 4,000 Hours it took to make eight different versions of the gown 10,000 Swarovski crystals hand-applied to the dresses 20 Minutes it took to get the dress on 4 Inches one dress was shortened by to help Lily flee the ball at midnight Advertisement

The trickiest thing Lily had to do in the gown was dance with her prince, played by Richard Madden. They practised endlessly, but poor Richard kept treading on her hems. Of course, when it came to actually shooting, they were faultless.

While I was delighted with the dress’s reception, I was surprised by the obsession with Lily’s tiny waist. People seemed to think it was setting a bad example to young girls. Some even accused the studio of digitally altering it. Ridiculous.

The reason her waist looks tiny is because the skirt is so huge – it’s an optical illusion – and she’s wearing a corset. The film is set in the 19th Century and every woman in the film is wearing one, even the maids.

I understand people get upset when incredibly skinny models appear on the catwalk or in photos. But Lily is not a super-skinny girl – there’s nothing wrong with her body. Cinderella is a girl known for kindness and courage, so it’s sad people just kept talking about her waist size.

Despite the sniping, the dress was everything I hoped it would be.

Design: The dress, made of rare, light yumissima fabric, was based on 19th century royal portraits

Frilling: The petticoat had flounced frillsat the hem, with a second three-layeredorganza petticoat placed over the top

Team effort: The dressmakers work on the petticoat. In total, Lily worea chemise, corset, crinoline, petticoats and the dress – five kilos in total