The couple got engaged in November, but Lachey admits he'd been planning it for a while.
"You gotta take it in stages. If you set the bar too high then you set yourself up for failure," Lachey tells Entertainment Tonight of his proposal. "This was a carefully calculated plan. You have to set the romance, but you have to take it in phases."
So what did Lachey, 36, do when he presented Minnillo, 30, with an Asscher-cut diamond flanked with trapezoids in California last year?
He got down on both knees, a pose Lachey jokingly refers to as "full begging position."
While her fiancé may be a big planner, it sounds like Minnillo still has a lot of wedding preparations to take care of.
"Gosh, I gotta get started on that," Minnillo says. "So much to do!"
For now, the couple are spending time together off screen – and on screen as guest stars on Hawaii Five-0.