You will need a piece of card measuring the 12cm x29cm..
Now score along the long side at 6cm and 23cm on the long side and 6cm on the short side.
Then mark along the top and bottom at 11cm and 18cm.
Next mark along the Centre score line at 8.5cm and 20.5cm. Now score between the 11cm and 8.5cm mark and 18cm and 20.5cm mark as shown below.