Someone talk me through cleaning pee out of a mattress?

Blot as much as you can first, don't rub. Then spray it with a mix of white vinegar and water, roughly half and half, blot again. Once it's just damp, cover the spot in baking soda and leave it for several hours, ideally overnight, then hoover it up. That's what deals with the smell. If it's an old dried patch, you may need to go round twice. Don't soak the mattress, wet foam takes days to dry and goes musty.