Plenty of people don't know how :-) Someone else will be able to direct you to a description, but for starters - try just sitting on a hard chair or on your hands, rocking back and forth until you can clearly feel your sit bones. They sort of protrude down like a lumpy knob when you sit on a hard surface. It's good to get an idea of what they feel like before you start rolling around on your back :-D

I think I measured the distance between mine by rocking around on top of a sheet of paper on top of a yoga mat. But you can also do it by lying on your back, feeling with your hands and using a measuring tape, with help from a hubby or friend. Some bike shops have these squishy pillow things that take the imprint. It's not a very exact science, so I'd recommend you measure several times.

Many people find a harder saddle that fits well is more comfortable than a soft saddle. Sounds counter-intuitive, but having support exactly where you need it and nowhere else is a good thing.