Ahhhhhhh, I love the simplicity of white card with a touch of color. This has to be one of my favourate Christmas cards this year as its just so clean and effective.
I cut a square of card until it was the size I wanted to create the embossed door frame. Then working on a magazine I placed the piece of card down centrally on the inside front panel of the card and scored around it. The great thing about this is that you can just keep that piece of card as a template to make this card again and again.
For the inside panels again it was just a bit of playing around to get a layout that looks good to you. Cut two squares and two rectangles. When you are happy with the sizes just place them on a magazine, piercing mat or mouse mat and score around the edges of each square and rectangle. Place on the card with 3D foam, pop on a brad and a quick topper and volia a door is made :)