I used this method to antique some vinyl shutters I picked up from the big box store.
You will need:
Paint for the areas that will show through the cracks (latex, acrylic)
Paint for your top coat (latex)
I started my adding the color that will show underneath the antique crackle. I used several colors of spray paint, hoping to achieve a weathered wood look. I concentrated on the areas that would naturally weather
And then I decided that I wanted to add some unexpected weathering. You know, just because
Plain old, school glue
Nicely layered on, I used a brush to even it out