These cards are reasonably easy to construct. The card size we made in class is 14cm x 14cm but so long as you start with square cardstock, you can make it any size you want. The height needs to be double the width.
Cut two cardstocks 14cm x 14cm. Half each so you have four cardstocks 14cm x 7cm. Score and fold each one at 3.5cm from each end so when folded they meet in the middle. Flatten out and lay two cards next to each other in landscape orientation, then lay the remaining two cards on top in portrait orientation. Turn the fold back and you will see a square (in the outer corners) bordered by the score lines. Glue this square in all four corners being careful to line up the top and bottom cards. I've used Tombow Mono Multi glue for this as I find it quite a good glue and it has both a fine and wide applicator. Leave to dry for a few minutes, then carefully turn and fold along the score lines, discovering a new 'flip' each time you fold. Keep folding in the same direction as it's very easy to get confused. Decorate as desired being careful NOT to cover any score lines, otherwise your card wont work!