Brass Darkening Solution made by Historic Houseparts that is used by Jim on the iron furniture shown in the photos
I like to just pour a small amount in a dish, dip a small piece of scotchbrite pad in the solution and then gently go over the surface with the soaked scotchbrite. Don't go over the same area too much or it can plate out in a layer of copper. Just play with it. It's really pretty easy to use. For smaller parts, you can just submerge the part. After it darkens things up, just rinse and then apply a layer of light oil. I like to rub back slightly with fine steel wool or a piece of denim.