The couple - who got engaged earlier this month - first met at the Golden Globe awards three years ago when the 'Pirates of the Caribbean' actor swiped some fast food from the 'Witness' singer's table - but she thought he was ''hot'' so let him have the meaty treat.
She told talk show host Jimmy Kimmel: ''In-N-Out plays a big role in my life. We bonded over an In-N-Out burger about three years ago at the Golden Globes.
''He stole one off of my table, I was sitting with Denzel Washington he took it and I was like, 'Wait! Who- oh, wow, you're so hot. Fine, take it.'''
Flashing her pink diamond floral-shaped engagement ring, she continued: ''Then I saw him at a party [afterwards] and was like, 'How are those onions sitting on your molars?' He was like, 'I like you' and then, you know, cut to...''
The 34-year-old singer - who was previously married to Russell Brand - also admitted she dropped some hints when it came to her engagement ring.
She admitted: ''I voiced my opinions.''
But while the ring may not have been a surprise, the 'Chained to the Rhythm' hitmaker had no idea what Orlando was planning when it came to his proposal.
She recalled: ''It was Valentine's Day... It was very sweet. We went to dinner I thought we were going to go see some art, after dinner, but instead we pulled up to a helicopter.
''He asked me to marry him in a helicopter. We did land and it was like James Bond, everybody had an ear piece and we were rushed over... we landed on a rooftop [in Los Angeles] and my whole family was there, and all my friends. He did so well.''
The host quipped: ''Very Bachelor.''
Katy laughed: ''Yeah! I'm just glad I don't have to go on that show now.''
However, the 'American Idol' judge admitted the 42-year-old actor's proposal didn't quite go according to plan as he'd handed her a love note to read in a bid to distract her while he pulled the ring box out of his jacket pocket so he could pop the question.
But while she was reading the message, the ring box got stuck and Orlando - who has eight-year-old son Flynn with ex-wife Miranda Kerr - managed to rip his jacket pocket and accidentally smash a bottle of champagne with his elbow.
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