You need a digital camera and a vintage twin lens camera. The image is composed with the twin lens, and then the viewfinder is photographed with the digital camera.
Anyway, while browsing through the images on RedBubble, I got the idea to try this with my Holga and my digital point and shoot (which is the only digital camera I own).
I don't think these qualify as true "through the viewfinder" images... but I still think they're kinda neat. And even though I'm definitely a non-digital film girl, it WAS kinda cool to shoot with my Holga and see the results right away!
Here are the results of my experimenting the other day:
thanks for looking!