When booking your flights, the airport closest to Stewart is Terrace BC (YXT) with connections from Vancouver BC (YVR). If you are looking for a road trip to Stewart after flying in, you could also fly into Prince George (YXS), or Smithers (YYD).
Generally the same as flying from inside Canada, be sure to fly into Vancouver BC Airport (YVR), and from there you will make a connection to Terrace BC Airport (YXT).
Leaving the Terrace airport, you will travel North approximately 10 km (6 miles) to Terrace BC. From Terrace BC you will drive Northeast for 99 km (61 miles) on Highway 16 coming to Kitwanga Junction. From Kitwanga Junction, you will travel 152 km (94 miles) on Highway 37 to Meziadin Junction to turn off onto Highway 37A for 61 km (37 miles) bringing you to Stewart BC.