Step 2: How It Works
In the pictures you can see the card open, before it is glued. The slider moves through a gap between the flaps on the right and stays horizontal because of the T-shape. I rounded the corners on the T to make it easier to slide.
The window in the front should be a little smaller than the slider, so you never see the edges. The gap on the right should be a little bigger than the slider.
To make for a clean image on the front, you have to make sure the slider has the same color as the front, and that it has no decorations, otherwise you will see them move along with the slider. I just made it blue, and I left a clear strip in the front image without clouds.
Unless you really want your message to pop out, you have to make sure the background for your text also has the same color as the front. This also has to be a little bigger than the window, so you won't see any edges.
After you made all the pieces, the airplane (or car/boat/dinosaur/...) will be attached to the grey spot on the slider (see image). Make sure your airplane is bigger than the window, otherwise it can slide underneath the front part of the card.