Put all ingredients except the rum in a saucepan, bring to the boil and let it steep for about 10 minutes on a low flame. Then strain over a tea strainer into a mug, heat-resistant grog glass etc. and add the rum to taste.
Personally, I like it a little sweeter, so I add 2 level teaspoons of brown sugar. Honey, syrup, etc. will certainly work too.