"What I told them is, 'This is all gonna be over in a week. It's all gonna be over. Whether you win or lose, you need to have a plan for the morning after the show," Harmon recounted while sitting down with ET's Sophie Schillaci on Monday.
"Every song idea that comes into your head is a potential cut on your record that you're gonna put out one day," he added. "That conversation's coming. So be ready the day after your show, whether you win or lose."
Harmon -- who recently released his first album, You Got 'Em All -- won Season 15 of American Idolin April 2016, with fellow contestant La'Porsha Renae coming in as the season's runner-up. But even after the win, Harmon says that it's still a difficult journey to becoming a music star.
"If you end up in Nashville, you're not just going to walk into a publishing house and go, 'Hey, here's my [Idol] trophy. Please give me some songs,'" he said. "It will help you to a certain extent, but at some point, you're gonna have to take a pen and a piece of paper and write a song idea down and put it out into the world."
Harmon said he "wasn't necessarily worried about that" the night he won the show, and he wishes that he would have "been on top of it just a little sooner."
"I took, like, a week [off], not long, but like a week, and I could have been writing down some inspiring songs during that week," he lamented.
However, Harmon admitted that he actually found himself in a creative lull.
"The truth is, after I won Idol, I didn't have any good songs," Harmon shared. "So I spent the last two years writing the album, which makes it even more special for me."
Harmon's debut album, You Got 'Em All, is out now.