How do the rest of you clean a mattress from pee?

Blot up as much as you can first, press with towels, don't scrub. Then an enzyme cleaner (the pet ones are made exactly for this) sprayed on and left for the time on the bottle, blot again. A diluted white vinegar spray works if you don't have one. Once it's only damp, cover the area in baking soda and leave it for a good few hours, then hoover it up. That's what kills the smell. For an old dried yellow mark, a mix of hydrogen peroxide, baking soda and a drop of dish soap works, but it can lighten coloured fabric so test a corner. And get a waterproof protector on it after.