Blot the pee as dry as possible with paper towels or rags. If it’s already dry, move on to step 2.
Pour vinegar into the spray bottle and spray all over the stain to saturate. Let sit for 10 minutes. Run your ceiling fan or open window so air can circulate.
Blot with paper towels or rags to soak up the vinegar.
Sprinkle baking soda liberally all over and around the stain. Let sit for 3-4 hours.
Vacuum up the baking soda from the mattress.
Notes
If you have some really difficult stains, use an old toothbrush to rub the vinegar into them vigorously.
I highly recommend using a spray bottle for the vinegar. However if you just don’t have one, saturate a rag with vinegar and lay it over the stain to pull the odor out.
Some methods online tell you to dilute the vinegar. You can do that, and it will help with the vinegar smell. But it also dilutes the effectiveness in my experience.
For really stubborn stains and odors, you may need to repeat the process.