Half-fill a small pan with water, bring it to the boil, then lower the heat to minimum.
Pour enough sake into a tokkuri (sake jug/pitcher) to bring it to about three-quarters full. Stand it in the water in the pan for about 5 minutes until the sake is warmed to your taste.
Check the temperature by lifting the jug and touching the bottom; it is normally indented in the centre. If it feels warm to the touch, it is ready.