Pick over the fruit and make sure none of it is damaged or dirty. You need to ensure the fruit does not go moldy as it matures. Mix the fruit and sugar in a large bowl overnight until the juices start to run.
Mix in the rum and spoon into a special Rumtopf jar or tall preserving jar. Make sure the fruit is submerged beneath the liquid: cover with a sheet of wet greaseproof paper scrunched up and one or two tea plates to weigh the fruit down.
As the summer progresses, add more fruits and sugar in the proportions given above, stirring the contents gently to mix. Seal again with a clean piece of greaseproof paper and the lid. You can add more rum if liked after about 4 weeks; it is not essential, although it does give it a lively kick.