fill your mug about half full with rum
put in a heavy scoop of store-bought hot buttered rum or tom/jerry mix
stir to disolve mix
add water till 3/4 full
heat in microwave
add small scoop of ice cream and enjoy
If you're making a large batch or for a party, you can prepare it in a stove pot instead of the microwave. Make sure to pre-heat the mugs though and add the ice cream in the mugs, not the pot.
If it's a large party with women, start with a lower rum/water ratio until most of the women are drinking, then increase the rum ratio as the party progresses.