This began with an 11" x 4.25" strip of cardstock. This was scored at 5.5". I cut 4" off of what would become the front of the card. I cut a one inch strip off that piece (1" x 4.25") to use at the bottom of the card. The size of this opening can vary with the image that you wish to display.
Using a plastic sheet that came with cling stamps that I've stored elsewhere, I made a window that was 4.25" wide x 4" tall. I wasn't sure what height I would use, hence the 4" even though the finished viewing window is 3" tall.
I used Redline (double sided) tape to adhere the plastic to the wrong side of the upper part of the front of the card. With the card closed and a line of this tape applied to the top of the 1" strip of pink cardstock, I lined up the bottom pink border on top of the plastic, having the bottom edge meet the back of the card. Because the plastic window is cut shorter than the total card height, be sure to apply the adhesive to the top edge of the cardstock you use for your bottom border. You don't want to accidentally seal your card shut.
The greetings were done on my computer in a shade of brown that matched the cupcake wrappers in the printed paper. The stamped image of the baker is from Leigh Hannan Impression Obession #MC181 "Cupcake", the cupcake is from There She Goes "A Little Treat". I used Prismacolor pencils to color the image.
I was originally going to color this with Copic markers. I stamped the image on X-Press It paper with the intention of cutting it out and layering it on top of different paper. After deciding to use pencils instead, I cut the image as a rectangle, rounded the corners and shaded the edge since the paper is a different white from the cardstock inside.
I will do this again, possibly with a scene inside and the outside made to look like a window or a camera viewfinder.