Remove the bedding, make a cleaning solution (vinegar with water, 50/50%), dab liquid with paper towels, then spray your solution on the stains and blot out excess liquid. After that, sprinkle baking soda, leave it for about 10 hours, then vacuum up the soda. If there are old urine stains on your mattress, you’ll need to use the algorithm provided in this article above (or use this one).