How do you bend moulding around a curved wall?

Depends on how tight a curve. A fairly wide curve can usually be done with wood that has been soaked for a few hours, loosening it up and allowing it to be bent around the curve and fastened in place.

The other way is to cut vertical slots in the back of the molding every eighth or quarter of an inch. They go about half way through. This allows the wood to bend and will usually leaves the front flexible enough to bend. Again letting the wood get really damp helps.