"I actually had to sit on his knee I was so shocked," she said on CBS's The Early Show, while promoting her new Christmas album. Simpson added that he planned the proposal long ago, had already discussed it with her parents and "was a gentleman about it."
Simpson, 30, only began dating Johnson, 31, in May – but he's already become, as she put it, "the best man in my life. Sorry Dad!"
"The best thing about Eric is that he's just really thoughtful and he keeps me very centered," she said. "And always makes me feel like my feet are on the ground and is probably the most special part of my life."
And he proved his thoughtfulness, she says, by picking out the right engagement ring.
"I've already done the traditional thing so I just wanted to do something that was different," Simpson said of her three-stone ruby ring from Neil Lane. "It was all Eric's idea and he just wanted it to be special."
Simpson said she first saw the ring after Johnson proposed – but didn't try it on right away.
"I said yes and we both definitely were teary-eyed. I didn't even try the ring on until like, five minutes in," she said with a laugh. "I was so shocked, just kind of sat there and cried with him."
In short, she says with a smile, "He did good."