The main measurements to make note of are the following:
On both the longest edges crease both a 1cm and a 3cm line all the way.
Turn your card to lay horizontally and crease 1cm away from the left edge, then 3cm, 7cm and 9cm. The rest of your chosen width will be divided into two with a crease line half way along from top to bottom. On the wrong side of the card, pencil mark the acetate window space by drawing a 1cm line away from the side of the space between the 3cm and 7cm crease lines- then cut that window out. On the wrong side of the card also lightly pencil mark a line from top to the bottom at 13 cm - this is where the box section will align to and be glued down when it is folded into place.
Score all the fold lines. Cut along the longest horizontal edge score line from the right hand side to the 9cm vertical score line on both the top and the bottom 3cm fold line.
On the wrong side of the card glue your acetate over the gap cut already between the 3cm and 7cm vertical lines.
On the 3cm fold line along the longest edge of top and bottom cut up to the 1cm vertical fold line (hopefully the picture will help explain that better). At the 1cm, 3cm, 7cm and 9cm crease lines cut a slight v-shape inwards from the longest edge - this will allow the flaps to fold away more neatly.
Fold the 1cm vertical line over and consequently bring the box over with it to the 13cm light pencil mark and glue it. That's it! Go for your life and decorate as you like now.
Phew, those instructions were hard - it's much clearer to watch a video tutorial but these notes will help me remember exactly how I did it and they make sense to me - well, they do now; goodness only knows if I'll know what I mean in a year's time!