1. Pick half ripe guavas - fruit should be light yellow. Wash, cut up and put in a large pot. Add water till you can just see it through the fruit. Cover the pot and boil till the fruit is soft. Mash up the fruit till it is all soft and thick. Put a cotton cloth in a bowl and fill with cooked guava. Tie up the corners of the cloth and hang over a bowl for juice to drip out.
2. Measure the guava juice.
3. Add 1/4 cup of lemon juice to every cup of guava juice.
4. Measure the lemon/guava juice into a big pot. Don't cook more than 6 cups at a time.
5. Add 1 cup of sugar for every cup of juice eg. for 6 cups of juice you need 6 cups at a time.
6. Stir over high heat till sugar has melted. Boil fast till jelly starts to set.
7. Jelly will ready when a teaspoonful put one a plate sets into jelly. Test by pushing the jelly with your finger. The top will wrinkle.
8. Pour the jelly into very hot, clean glass jars. (heat jars by putting in a low oven for 10 minutes or boil in water.)
9. Cover the jelly with lids which have been boiled clean by boiling.