I use crackle medium all the time on my spooky houses. I may be addicted to it.
I put down a dark color first (usually black). When it's dry I use the crackle medium (got a fairly large bottle for just a few $$ at Hobby Lobby). Paint the crackle on fairly thick for big cracks or lighter for finer cracks. Then after that's dry I use a lighter color. The heavier you paint the light color, the better your crackles will be. But you have to work fairly fast because the crackle medium makes the paint dry really fast, and if you try to go over it once it's started to crackle it will smear.
Hope this helps!