The famous dress's flowing 25ft train meant that it had to be moved from his shop to Buckingham Palace where it was kept in a cordoned off wing to keep the design a secret.
"I said we can't have these [footmen] here because they're going to see what's going on and they might leak it and we've promised to keep it a secret," the 61-year-old explained.
He spoke about his first impressions of the Princess when they met through a chance meeting with an editor of Vogue.
"She was a very nicely brought up, pretty girl, quite shy," he said. "She said, 'I'm looking for a pretty dress'."
David, who worked with his then wife Elizabeth to create several dresses for Diana before her wedding day, was speaking to fellow I'm A Celebrity... contestant Westlife's Kian Egan.
The designer was the name behind the iconic strapless black gown she wore to her first public engagement at the Royal Opera House. It hit headlines all over the world and months later she asked him to design her wedding gown.
"I picked up a call in the studio and she said, 'It's me'. She never said 'Lady Diana'. I always used to say, 'Hello, Me.'
"She said, 'I'm just wondering, would you do the honour of designing my wedding gown? I thought, Christ, perhaps it's a hoax call...
"All this press that she's getting married and nobody knows anything and then she rang up and I said, 'Yeah!'
"When she came in I said, 'Did you call me?' 'Yes', she said and started laughing. Immediately it was announced we got surrounded by press, the world."