Cut your A4 sheet to exactly 11 1/2" - so just a tiny bit to cut off.
Score along the longest side first at 1 1/2" - 5" - 6 1/2" and 10".
Turn the cardstock so the shortest side is at the top and score at 1 1/2" from the edge on both the left and right hand side.
Crease along all of these lines.
Make four cuts along the score lines on each of the long sides just as far as the 1 1/2" score line.
The box should start to take on a life of its own and you can see the shape appearing.
At this stage I would decorate the lid and sides whilst it is still flat.
Once happy with my decoration I would glue the short tabs and fold them into place on the inside of the box.