The method is fairly easy: cut two pieces of card the same size, and a piece of acetate the same height but 3cm narrower. Score two vertical lines in each piece of card: one 3cm in from the left edge and one 3cm in from the right edge. Similarly, score two vertical lines in the acetate 1.5cm in from the left and right edges. Fold all the edges in. The acetate sits between the two pieces of card, which form a "box" around the acetate. The front piece of card has an aperture cut into it. This gives three layers for decorating. Wait until after decorating to glue the edges together. Glue the acetate edges to the inside of the back piece of card. Glue the edges of the front piece of card around the edges of the back piece of card. PVA dries clear and provides a good, strong bond.
The mermaid card uses a free image from the Graphics Fairy's blog for the front of the card, and the aperture was cut with a craft knife. Fish were stamped onto the front of the acetate using archival ink, and gel pens used to colour the back of the acetate. The back of the card has a piece of blue holographic card added to look like water, and more fish were stamped onto the holographic card and coloured as before.
The aperture for the fish card was die-cut and a scene created in a similar way (it's a shame but the photo doesn't show the fish very well). This time, a Martha Stewart deep edge punch was used to create the "fronds" from different coloured scraps of card. A couple of extra fish were stamped onto acetate and cut out, then placed as if swimming into and out of the fronds.
The meadow card was created in a similar way, but the background was formed from cream card inked & stamped with distress inks, then the scene created with a sticker sheet. The sentiment is from docrafts and reads "Watch where you tread in life as true happiness can be found growing at our feet". These cards are great for using up those sheets of stickers you've had lying around for ages :)