and Nickelback's Chad Kroeger opened up to Ryan Seacrest
on Tuesday about their engagement and upcoming nuptials
– where the singer revealed the groom is in the dark about some of the wedding details.
"It's going to be quite spectacular, and there's a theme to it," Lavigne said during an interview
on KIIS-FM's On-Air with Ryan Seacrest
Tuesday. "We're going pretty big, but Chad doesn't even know everything."
Lavigne demurred when Seacrest pushed for details on the date, but she did reveal that the roots of their love affair, which began in the studio while Kroeger worked with Lavigne on songs from her forthcoming album, will play a role in the nuptials.
"I'm going to take the lyrics from the sheet music from one of our songs and wrap it around the wedding cake," she told Ryan, while Kroeger gave a look of genuine surprise.
As for the details of their engagement, Lavigne explained: "I had been making this scrapbook of my time in the studio. I walk over and he had taken a photo of himself holding the ring box and put the Polaroid in my scrapbook. Because I had stickers everywhere he put in 'Will you marry me?' It was really romantic and kind of made sense."
Kroeger, who presented her with a 14-carat diamond ring
, admitted the proposal made him more jittery than a big performance.
"You can travel the world and sing songs to thousands of people and not have your heartbeat raised at all," he told Seacrest. "You get down on one knee and commit yourself to a woman for the rest of your life. I was very nervous!"
That makes sense, considering his focus these days seems to be on his bride-to-be
. "I try not to take too much interest in the world's opinion of our relationship," Kroeger told PEOPLE last fall. "I just know the both of us are very happy."