To eliminate dog urine from a mattress, begin by blotting up as much urine as possible with a clean towel. Mix an enzymatic cleaner with water according to the manufacturer’s instructions and apply it to the affected area, allowing it to sit for the recommended duration. Blot away the solution and repeat the process until the urine is removed. Sprinkle baking soda onto the area and let it sit for several hours or overnight to further absorb odors. Vacuum up the baking soda and use a black light to check for any remaining stains. If necessary, repeat the enzymatic cleaning process. To prevent accidents, train your dog to avoid urinating on the mattress and consider using a waterproof mattress cover.