Rough up the existing finish with #000 steel wool. Wipe all the dust off the surface with a lint-free cloth and apply a water based primer using a disposable foam paint pad or disposable brush. Allow two hours drying time before moving on to the next step.
Apply a base coat of flat or eggshell acrylic latex paint using a disposable foam paint pad or disposable brush. Eventually, the color of this paint will show through the cracks, so it's important to choose carefully. You can achieve a dramatic effect by selecting a color that will contrast sharply with your top coat. If you are using brown colored paints, applying a darker version of your finish coat can help create the illusion of aged varnish. Follow the paint manufacturer's directions for drying times.
Roll on a thin layer of Elmer's Glue, liquid hide glue or a similar adhesive product. Applying a thin layer of glue with a foam mini-roller creates a relatively uniform pattern of fine cracks. If you apply a thicker coat of glue with a brush the cracks will appear deeper, and uneven brushstrokes will increase the randomness of the crack pattern. This is an inexact science, however, and the results are not completely predictable. Let the glue dry for one to two hours before moving on to the next step.
Apply a finish coat using a water based paint. A tinted PVA paint will normally crack as it dries over the glue layer. If applied normally, a casein-based milk-paint will usually form small cracks as it dries, or form deeply cracked chunks if applied as a thicker coat.
Allow the crackled finish to dry overnight. Then apply two coats of a matte-finish water-based polyurethane varnish as a protective sealer, using a high-quality synthetic fiber paint brush in order to minimize brush marks.