Brass and leather make a nice combination, but brass can tarnish and leave green tarnish stains on your leather. Tend to tarnish stains as soon as possible so they don't have time to work their way down into the pores of the leather. Using supplies from home, you can remove brass tarnish stains without damaging the leather.
Mix white vinegar and baking soda in a bowl to create a thin paste. The mixture will foam a little. The vinegar and baking soda mixture creates a mild acetic acid which helps remove the brass tarnish.
Scrub the paste into the brass stain using an old but clean toothbrush. Use a gentle scrubbing motion, working the paste into the pores of the fabric. Allow the vinegar paste to sit on the tarnish stain for a couple minutes.
Dampen a cotton cloth with water. Wipe away the vinegar paste and the green tarnish stains from the brass. If any tarnish stain remains, repeat step 2.
Dry the leather thoroughly, then apply a small amount of leather conditioner with a clean cotton cloth and rub it into the leather.