Take a piece of double sided designer series paper and fold in half (right side shown in first picture). Mine is 28cm x 14cm.
Open the paper and taking the right hand side fold in to the centre.
Open paper out and then taking the left hand side fold upwards using the side fold as a guide. Unfold and repeat step but folding down.
Open paper again and mark the centre of the left hand side at 7cm. Draw lines from the centre to the side fold lines, and the cut.
The piece should easily fall in to place, only requiring a small piece of adhesive to hold.