Patina black is a liquid that you brush or rub onto solder or lead to chemically change the colour from silver to black. Note that the solder needs to be fresh, or well rubbed with fine steel work for best results. Transfer from the bottle to a small container like the cap and dip your brush into that. Do NOT keep dipping your brush into the full bottle or you will contaminate it.
Apply fairly liberally, leave a few minutes and then blot dry with a paper towel.
It is best to polish the patina after it has fully dried with a wax polish to make it nice and shiny and preserve the look. We use a car polish (must NOT contain cutting compound) but even something like Pledge furniture polish is better than nothing.
We also have copper patina here