On NGS group, we are studying Julia Andrus' book "Paper Transformed". This week we are looking at the section on White Glue Crackle. Easy & inexpensive way to make crackle, just takes a bit of time.
Start with card stock, using a color that is your desired under tone color. Paint a thin application of white school glue over the card stock and let dry until it is tacky, but not dry. Julia mentions to created fine line crackle, use the glue straight. For wider cracks in the crackle, thin the glue with water (which is what I did).
When the glue is tacky, paint over the paper with acrylic paint of your choice of color and let the paint dry naturally (this is where the time comes in).