Once you get the hang of these and find what method works for you, they're a piece of cake to do! They look hard, but in reality, they aren't. After a few tries, it doesn't take very long.
Once you have your base color, take a dotting tool or bobby pin and make splotches or dots of brown (or the color of your choice) where you want them. They don't have to be perfect. Once you have splotches of brown on all of you nails, take a smaller dotting tool or tooth pick and start putting black around the brown. On some of the spots you can make a C shape, on others you can make a ( ) shape. Once you're satisfied with your spots, add some small black spots in areas where you have the room. This will make them look more realistic.
If the way I tried to explain it isn't that clear, cutepolish has a really good tutorial for how to make the spots. http://www.youtube.com/watch?v=Dg6_atTygco