Fill a pot with water and put it on high heat.
Meanwhile, wash and carefully peel the guavas, then cut them in half.
When the water boils, add the guavas carefully and cook for 15 minutes, or until soft.
Take them off the heat and discard the water. Blend the guavas for a few seconds only: the seeds are hard and grinding them makes the paste gritty.
Pass the blended mixture through a strainer to get the pulp and leave the seeds behind.
Put the pulp in a pot with the cup of orange juice and the sugar. Cook over low heat, stirring frequently with a wooden spoon so it does not stick to the bottom.
Cook until it becomes a smooth, glossy paste. The test: if you can see the bottom of the pot for a moment as you stir, before the paste closes over it, it is ready.
That takes about an hour.
Turn off the heat and let the mixture cool a little.
Pour it onto a tray and spread it out level.
Dusting the tray with sugar first stops it sticking, and you can sprinkle more over the top.
Let it cool for several hours, ideally overnight. Once it is firm, cut it into bars. Cutting it warm gives you a sticky mess rather than clean slices.