Dice guavas into one inch cubes. Place in a large saucepan and cover with just enough water to cover the fruit well. Cover and cook till guavas are very tender, about half an hour. Cool. Cover a large bowl with a muslin cloth and invert fruit onto the cloth taking care to catch the liquid that drips through the muslin, into the bowl. Gather the four ends of the cloth and tie a knot and hang this'bag' for about four hours and collect all the drippings into the bowl. LIGHTLY squeeze out any juice left in the cloth. Discard pulp. Measure the liquid. For every cup of liquid add one cup of sugar and one tablespoon of lemon juice. (Eg. if there are four cups of liquid you would need to add four cups of sugar and four tablespoons of lime/lemon juice.) Put this mixture back on fire and bring to a boil. Lower heat and cook, stirring constantly (as the liquid tends to boil over), till the liquid starts coating the spoon thickly and the juice/jelly drips from the spoon in jointed drops. Let stand for half an hour and pour into jars while still warm. Cover and seal.