Tape around all woodwork and windows, and remove all switchplates, outlet covers, light fixtures, or anything that attaches to the wall. Venetian plaster is thick enough that precise application is difficult, so you don't want to put yourself in a situation where you're trying to dab it into tight spaces while maintaining a clean edge. Give yourself room to be expressive. Wash down the walls with a clean, wet rag, and let it dry completely.