"Knocked it out of the park," Stewart said of Meyer. "I scored."
Fallon told the Spencer star, "It's the best thing in the world. I can't even tell you. Now you have a buddy who kind of has to do things with you."
"I know," Stewart agreed. "And we have to help each other out with stuff, too. You gotta do the stuff I don't wanna do, man."
Stewart additionally appeared on Thursday's episode of the Todayshow, and revealed that she and Meyer got engaged "months ago."
"The funny thing is even Dylan's dad sent us an email congratulating us," she shared. "He was like, 'Girls! So happy for you.' [We were like,] 'You were at the engagement party months ago.'"
As for how she feels now that she's engaged, Stewart said it's all "perfect."
"I feel so lucky," she said. "It's so nice to know something in this world. I'm so happy. I'm so stoked."
After previously joking that she'd like Guy Fieri, who she called a "sweet, sweet spikey-headed man," to officiate her nuptials, the morning show played a video of the Food Network star agreeing to the gig.
"I've heard from the Flavortown grapevine that you're looking for a sweet, spikey-haired officiant for your wedding," he said. "I'm all in."
"Oh my God," Stewart excitedly reacted, revealing that she "absolutely" wants to take him up on it.
"Do you know where he lives? What's his address?" Stewart jokingly asked. "I'm like, 'Does he live in L.A.? We should talk about this.'"
Stewart first confirmed her engagement on Sirius XM's The Howard Stern Show earlier this week.
"We're marrying, we're totally gonna do it," she told Stern. "I wanted to be proposed to, so I think I very distinctly carved out what I wanted and she nailed it. We're marrying, it's happening."
The actress also shared details about what she wants their wedding to be like.
"The food is the biggest deal. I don't care about flowers and s**t like that. The food needs to be on point," she said. "I want to wear the best pair of Levi's ever and, like, an old T-shirt that has a tuxedo printed on it, but like, cut-off and barefoot. I want to get married in, like, Topanga. I want to be chilling. I want it to be very homey. I want to see her laugh hysterically when she sees me in, like, a faux tuxedo T-shirt."
"I want to stay home," she added about the location. "I want to be in L.A .so everyone can come. I want it to be pretty chill. I don't want anyone walking anyone down any aisles. We're just going to, like, stand and do vows and f**king party after. It's kind of just a great excuse to get together and say 'I love you' in front of all your friends."
ET recently spoke to Stewart about playing the late Princess Diana in Spencer, and what it was like portraying a devoted mom of two. Stewart said she wanted kids eventually, but for now, was content with her dog.
"Well, I have a dog and I am sure that when I have children, ultimately, I will realize that I have room to grow -- but I think that this is tippy top... I do love my dog quite a lot," she said. "For me, at this point, it is a pretty happy place."