Wash 1 kg of ripe guavas thoroughly and chop them into small pieces, discarding the seeds.
In a large, heavy-bottomed pan, add the chopped guavas and 1 cup of water. Cook on medium heat for 15-20 minutes, or until the guavas are soft and mushy.
Use a sieve or a muslin cloth to strain the cooked guava pulp, collecting the smooth puree in a bowl. Discard the seeds and skin.
Measure the guava puree and for every cup of puree, add 3/4 cup of sugar to the pan.
Cook the guava puree and sugar mixture on low heat, stirring continuously to prevent it from sticking to the pan. Cook for 20-25 minutes, or until the mixture thickens and reaches a jam-like consistency.
Add 1 tablespoon of lemon juice to the jam and mix well. This will help to preserve the jam and enhance its flavor.
Allow the guava jam to cool completely before transferring it to sterilized glass jars. Seal the jars tightly and store them in a cool, dark place.