1 pound guavas
Wash guavas and cut them into 1/4-inch slices. Put them into a non-reactive heavy-bottom saucepan and barely cover them with water. Bring the liquid to a boil, lower the heat, and simmer for 20 minutes or until the guavas are tender. Pour the contents of the pan through a strainer lined with cheesecloth. Let it drip for 30 to 45 minutes until most of the liquid has gone through. Squeeze gently to extract any extra liquid.
Measure the guava juice and return it to the saucepan. For every cup of juice add 3/4 cup sugar and 1 tablespoon grenadine. Stir well. Bring to a boil and simmer, stirring, for about 20 minutes or until the jelly reaches 220 degrees F on a candy thermometer. Ladle into hot sterilized jars and seal.