Wash the fruits and cut them into quarters, unpeeled.
Add citrus to a cup, with some sugar. Smash them together with a pestle or something similar, to squeeze the juice into the sugar.
Add ice and rum or vodka, in this order. Mix it well together.
Adjust sugar as desired, [optional] water it down a bit if too alcoholic for your taste.
Note: I think that the traditional version uses rangpur, but limes as in the pic are also extremely common. Some people (incl. myself) use vodka instead of rum.
It is from the Caribbean, but it's fairly popular also in South America. (And also in that chunk of North America between Panama and Mexico, incl. both.)