Depending on the type of paint you are using there are a couple of ways of doing it.
If you are brushing a 'household' paint, like you use for wood and metal in t'ouse, Get to the paint store and by some Gummy Arabicum. Its a clear 'paint' that you paint onto the base colour- When dry paint the final colour over it.
Tip: If you paint the arabicum left to right, paint the top coat top to bottom, so the brush strokes cross, it works better that way. Also a thick layer of Arabicum will give large cracks and flakes, a thin layer loads of fine cracks and flakes.
If you would rather spray, you can actually by rattle cans of crackle paint.
Ian M