Determine the type of patina you want to achieve. Consider that copper, bronze, iron and brass all age in different ways: Copper becomes green, bronze and brass become dark, almost black-looking, and aged iron can look reddish or black. Purchase the type of patina powder you need to buy for your project.
Go to your nearest art supply store or search online for the readily available patina powders that are suitable for your project. Speak to a knowledgeable salesperson at an art supply store if you've never applied patina powder before.
List all of the other items you need for your project. The exact list depends on the finish you're trying to create, the surface you are aging and what patina powder product you purchase. Assemble everything you need before you start, as the patina process can be complicated and involve several steps.