Japanese sake is an alcohol which can be enjoyed at various temperatures, but sake that has been warmed, in particular, is called kanzake. It can also be called okan, as well as other names.

The fundamental method for okan

1. Boil water in a pot and then turn the heat off.

2. Put the Japanese sake into a heat-resistant container, and dip it into the boiled water so that the pot’s water does not enter the container.

3. Bring it out once the sake has reached your preferred temperature and enjoy.

A heat-resistant container could be a tokkuri (primarily something like a small pottery or ceramic decanter) or a chirori (metal containers used to heat sake). Using either of those specialized pieces of equipment would be handy, but using a common mug can also suffice. You can use a microwave to heat up your sake as well, but using boiled water in the okan fashion makes for a richer flavor.

A referral website on how to make okan sake

http://www.japansake.or.jp/sake/english/howto/temperature.html