Place a small plate in the freezer. If you plan on canning your jam, sterilize jars and lids. 

Prepare the fruit: Place watermelon in a blender and puree. Coarsely chop pineapple, mango and dragonfruit. Zest the orange, then peel and coarsely chop the fruit.

Place all of the fruit in a large pan and cook on medium until the fruit is softened. It will take about twenty minutes, depending upon how much heat you use.

Put the cooked fruit in a food processor and process until there are no large chunks remaining. Don't overprocess, though, as you want to keep some texture. 

Measure the fruit and add 3/4 as much sugar as you have fruit. For example, if you have 4 cups of fruit, add 3 cups of sugar.

Place the fruit and sugar back in the large pan and cook over medium heat, stirring, until it comes to a full boil. Add the pectin and stir constantly until it returns to a full boil.  Continue stirring for one minute.

Remove the pan from the heat. Spoon a little of the jam onto the plate that has been in the freezer. Return to the freezer for two minutes. The jam should appear thickened and should wrinkle when you touch the surface. 

If the jam does not seem thick enough, return it to the heat and boil for another couple of minutes. Repeat the freezer test. 

The thickened jam is ready to be placed into the sterilized containers. Process for 10 minutes. Remove jars and place on a towel. Do not retighten the rings. Leave jars undisturbed for 12-24 hours.