"He called my dad and got his blessing first," Lambert tells PEOPLE about Shelton's surprise proposal on May 9. "That was so Southern and old-school and perfect."
Equally perfect: The diamond-and-platinum ring "Hillbilly Bone" singer Shelton, 33, chose on his own.
"I was so proud of him," says Lambert, 26. "We've been together five years, so he knew exactly what I wanted but it's so much more perfect than I could have picked out myself."
The setting for the proposal was distinctly down-home – in the woods near Lambert's Tishomingo, Okla., home. And the celebration was similarly informal: "We had a couple of Bacardi and Diet Sprites – in a Solo cup!"
The engagement is yet another high point in a stellar year for Lambert, who in January scored her first No. 1 single with "White Liar" and in April won Album of the Year and Top Female Vocalist at the ACM Awards. Tuesday morning, Lambert was nominated for two CMT Awards, while her groom-to-be also received a nod.
"2010 will not be forgotten in my world," she says.
And the best may be yet to come. "Weddings are great and engagements are great and diamonds are especially great," Lambert says. "But I'm just excited to be married because I want to tell Blake I really can love him the rest of my life and be happy and make him happy."
For more on Lambert and Shelton's engagement – and a photo of the ring! – pick up the new issue of PEOPLE, on newsstands Friday