Hi, Ellen. The traditional way to do this requres a coat of hide glue under the paint. The problem with it is that it is not very controllable, and even less so with the many paints that are available and on the market today. I suggest that you really test what you plan to do well before you try it on the pieces that you want to have this finish. It is usual to apply a clear finish over the color to kind of bond the flakes together and so they don't flake off. Polyurethane is usually used for this. Make this part of your testing program so you don't end up with any surprises.
You might also check the craft stores since I believe that they have spray paints that are made to crackle.