You need 2 square 21x21cm.
Mark the folds.For the bottom of the box 3.8cm from the edge on all sides and the lid of the box mark the folds to 4cm from the edge.
At each corner cut a small v-shaped notch at the point where the lines intersect.
To assemble the corners to bring the flaps inside the box and paste.
Once the box assembled craft your decoration.
I used the stamps of new kit july "Life's a beach".