I haven't made one of these cards for ages and I am grateful to Wendy Cranford and her YouTube video for a reminder of how to make one. I adapted her measurements to fit the stamp that I wanted to use and ended up with an A5 card.
I started by creating a blank for the 'magic' pocket. A piece of A4 card was folded in half then trimmed to make a 17.5cm x 12.5cm landscape card blank. Using the fourth largest die from the Double Pierced Background Rectangles set by Sue Wilson, I cut a rectangle from the front of the 'card'. The huge happy birthday stamp by Woodware was stamped onto the back panel through the aperture using Jet Black Archival ink and the Tim Holtz stamp platform to ensure precision. A piece of acetate cut to the same dimensions (12.5cm x 17.5cm) was placed over the image stamped on the card and stamped in the same way. This ensured that the acetate and card stamped images would be perfectly aligned.