The crackle medium is applied using a soft brush, once the 1st coat of paint is dry, to the metallic silver and blue squares. Wait about ten minutes for the medium to start drying. Repaint the squares using a contrasting color (for example, a slightly deeper shade of sky blue compared with that used for the left-hand side of the canvas, if you've any left, you can simply mix in a little more blue). It's important to apply the paint in one brushstroke, do not bring the paintbrush back over an area that has already been painted (otherwise you will cover up the cracks that are in the process of forming). Leave to dry. The cracks will start to appear.