Fully ripe watermelons that have a soft red flesh should be used for making jelly. The juice is extracted from the fruit and used to make a jelly, rather than a jam (jams contain pieces of fruit pulp whereas jellies are made from fruit juice). Watermelons contain little natural pectin so pectin has to be added to ensure the jelly will have a good set. Other fruits that are high in pectin, for example apple, rind of passion fruit, can be mixed with the watermelon juice if commercial pectin is not available.
Watermelon juice is not very acidic (pH above 5.0) which is too high to make a good jam or jelly. Jams give a gel when there is the correct ratio of pectin to water and the pH is between 2.5 and 3.45 pH.The optimum pH to give a good gel is pH 3.0. Therefore citric acid or lemon juice has to be added to the recipe to reduce the pH and increase the acidity of the juice. The yield of usable fruit from the whole fruit is approximately 43%.
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sources: practicalaction.org, pickyourown.org