"Cracked" or "crackled" paint finishes are achieved by producing cracks in one layer of paint, which exposes a second layer underneath.
The Matisse cracking process relies on a reaction between the topcoat of Matisse Background Colour and the Cracking Medium which has been applied below it. Once the topcoat is applied, the cracking coat below becomes semi-liquid again and shrinks, taking with it the topcoat.
It is worth noting that the topcoat should be applied systematically (within 12 hrs of the Cracking Medium drying). When applying the topcoat, it is important not to re-work areas that have already been covered as re-working an area can lift the cracking coat and stop the process.
The thickness of the topcoat will dictate how big the cracks are. The thicker the topcoat the more easily it is able to hold together and resist cracking but when the Cracking Medium gets its way and manages to crack the topcoat, it will result in a very large crack. Thus the thicker the topcoat of Matisse Background Colour, the fewer the cracks but they will be big ones.
Conversely, if the topcoat is applied very thinly, the Cracking Medium will encounter less resistance from it and therefore result in many more cracks but they will be much smaller in size.
Note: As with any technique, if you've never tried this before it is advisable to practise on some cardboard or scrap material first. The process is fairly straight-forward, but requires some attention to the guidelines.