In a high ball glass or something of similar volume (you need a big glass for this drink)
Place the mint leave, lime pieces and sugar in the bottom of the glass in that order. Muddle with the blunt end of a rolling pin, until the flesh of the lime comes away from the rind but the mint leaves are still intact. Pour on the tequila. Stir. Add ice to the top then top up with soda. Stir again gently.
If you would like a salt rim, it is easier to muddle in one glass then pour the muddled ingredients into a glass that has a lime rubbed around the edge and dipped in the salt rim, then add the rest of the ingredients.