You pour some water in a kandouko, and place burning charcoal in the brazier in this device. The heat of the charcoal warms up the water. So, you put a flask or another vessel containing sake in the warmed water and wait. Soon, the sake becomes warm.
(For details about the kandouko, see this post: "Finally got a kandouko (sake warmer)!"
The kandouko is not merely a sake warming device, but it also serves as a tiny hibachi. You can set a grill on the brazier and cook some food while warming and sipping your sake. You can grill dried fish, shiitake mushrooms, and other foods. This way of drinking may look somewhat poorish, but it is folksy, and I feel like I can flatter myself that I am a bearer of sake culture.
Lately, I am practicing a new way of using my kandouko; I warm canned food on the brazier
If you are interested in warm sake, how about getting a kandouko?