From Santiago
You can take a 3h20min flight or a 4h flight depending if it stops over Puerto Montt or not. There are 3 airlines with daily flights. The take a 3 hour bus trip to Puerto Natales.

From Puerto Montt
You can take a 3 days amazing trip through beautiful channels and fjords with romantic landscapes aboard Navimag ferry, see virgin forests and patagonic fauna.

From Buenos Aires / Rio Gallegos
First, take a 3-hour flight from Buenos Aires to Rio Gallegos and then a 5-hour bus trip through the Magellanic Pampa, a beautiful, lonely a thoughtful place. It is about 260 kms but customes to go through. Then in Punta Arenas take a bus to Puerto Natales. (3 hours)

From Ushuaia
The easiest way: an airplane. You can also take a 10 hours bus ride or an amazing navigation tour which takes 4 or 5 days with entertaining activities, zodiac excursions, all inclusive system aboard Mare Australis or Via Australis. Then in Punta Arenas, take a 3 hour bus trip to Puerto Natales.

See a Google Map of Puerto Natales.