Shelton, 33, popped the question Sunday near Lambert's home in Tishomingo, Okla., Lambert told the magazine.
"I was so proud of him," the 26-year-old Lambert said about her diamond-and-platinum engagement ring. "We've been together for five years, so he knew exactly what I wanted but it's so much more perfect than I could have picked out myself."
Shelton and Lambert are both on the road touring. On Tuesday, Lambert received two CMT nominations, and Shelton one.
"Weddings are great and engagements are great and diamonds are especially great," Lambert said. "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."