There are several methods.

• Peat (turf) burns well and is used to dry grains. These grains will give a smoky flavor, and also a earthy aroma -- since peat comes from the ground and is like soil.
• The inside of the barrels have char in them. And some whiskey's are matured in barells previously used for peated whiskey.
• You can also smoke whiskey, by using a smoker.

Cheers!