Crackle is a 2 step process using incompatible paints. When they are apply together they react against each other and try to pull apart - creating the cracks.
To ensure a good crackle pattern, make sure all porous surfaces are first sealed with a primer. Plasti-kote Super Primer is ideal because it dries quickly and allows you to quickly continue with the Crackle finish base and topcoats.
For an effect with small crackles, spray 1 coat of the base, wait until the base is tacky and then apply a light top coat.

For a medium crackle effect, spray 1 coat of the base, wait until the base is tacky and then apply a medium coat of the top coat.

For a large crackle effect, spray 1 coat of the base, wait until the base is tacky and then apply a heavy top coat.

When applying the topcoat, the can should be held only 6-8 inches from the object. This is much closer than applying a typical spray paint application, so be careful not to get too close in order to avoid runs and sags.

If you are going to use Crackle again, there is no need to remove the current coating. Begin again by applying several basecoats over the original top coat. Allow to dry then apply the topcoat following the same instructions as stated above.