Start by washing 1 kg of ripe guavas. Cut them into quarters without peeling.
In a large pan, add the guavas and enough water to cover them. Bring it to a boil and then simmer for about 45 minutes until the guavas are soft.
Strain the mixture through a muslin cloth or a fine sieve. Do not press the pulp, just let the juice drip naturally.
Measure the juice. For every cup of juice, add 3/4 cup of sugar.
Pour the juice and sugar back into the pan and heat it on a medium flame. Stir continuously until the sugar dissolves.
Once the sugar is dissolved, increase the heat and bring the mixture to a rapid boil. Continue boiling until the jelly reaches the setting point. To test this, put a little jelly on a cold plate. If it wrinkles when you push it with your finger, it's ready.
Once the jelly is ready, remove it from the heat and let it cool for a few minutes. Then, pour it into sterilized jars and seal them while still hot.