The quality of the watermelon used is important. It needs to be ripe and sweet- the kind that releases its aroma as soon as you slice it and the flesh of which seems to dissolve when eating it ( as opposed to the fibrous, light pink and watery varieties). I personally believe that it benefits the fruit if it is well chilled prior to slicing it. If you have a cold brook to lay it in for a while, even better. The rest of the ingredients also should be chilled well, but pay attention to the anise liqueur as it tends to crystalize at too low of a temperature. I am sorry that I am not able to submit along a photo, but the watermelon season is not quite here yet. —Droplet
Chill watermelon and seltzer well. Slice the watermelon, deseed it and puree in a blender enough of the fruit to get 3 cups. To that add 1.5 cup seltzer and strain through not too fine a sieve (optional). Pour the strained juice into a jug for serving or a designated glass bottle. Add the rest of the seltzer and the liqueur to taste. It is present here for the anise flavor and the whiteness it imparts, and not in a ratio that will make it an alcoholic drink.