Lime and spearmint muddled with sugar in the glass, then ice, soda water and white rum. Two glasses in five minutes, and the muddling is what decides whether it tastes of mint or of grass.
Press the mint, do not tear it. Muddling is meant to bruise the leaves just enough to release their oils against the sugar and the lime. Grind them and you break the leaf tissue itself, which releases chlorophyll and turns the drink bitter and grassy. A few firm presses with the end of a wooden spoon is all it takes. Hierbabuena is spearmint, which is what Cuba uses and what a mojito is supposed to taste of. Peppermint is a different plant with far more menthol in it and it overwhelms the rum. If the packet says mint without saying which, it is almost always spearmint. Soda water and not tonic. Tonic contains quinine and sugar and gives you a different drink entirely. The rum goes in last in this version, after the soda, which is unusual but keeps the fizz.