Apply Venetian plaster to the wall with a trowel. Use large, overlapping strokes, and apply a thick layer of plaster. Feather some edges, but leave the overall look messy and random. Allow to dry until the plaster no longer feels cool.
Apply a second coat of plaster the same way, being careful not to fill in the roughness of the first coat. Keep it messy and random. Allow to dry for about 15 minutes, or until the plaster starts to feel leathery instead of smooth and wet.
Trowel back over plastered areas, varying your stroke, pressure and the angle of the trowel. This will lift the plaster from some areas and redistribute some plaster to other areas. If you're not achieving the chipped, ragged effect, allow the plaster to dry a bit more and try again.
Pounce over the wet plaster with a stipple brush or a coarse-bristled broom head to add additional texture, if desired. Use firm pressure for more obvious texture. Imprint with coarse sandpaper for an old stucco look.
Repeat the process with another coat of a different color if desired, allowing the previous color to show through. Allow to dry completely.
Use a flimsy, flat Venetian plaster burnishing trowel to burnish the dried plaster to add shine and scuff marks if desired. Use firm pressure and a rapid circular motion. This will accentuate the roughness of the finish, bringing out the differences in depth.
Apply a coat of Venetian plaster protective top coat as directed on the can. A polish-type product will add shine, or a paint-on product will preserve the existing sheen.