First, make sure you pick a good watermelon. You want the surface of the watermelon to be firm, and when you knock it, you should hear a hollow sound. A dull thud means that you've got yourself an underripe (aka: not very sweet) melon. Furthermore, the melon's field spot - which is the spot the watermelon was resting on as it grew - should be a creamy yellow. Basically, the darker the yellow of the field spot, the longer the melon has had time to ripen on the vine. But steer clear of melons that either have no visible field spot or a very light colored field spot (whitish); they're not ripe. And if you really want to make sure you get yourself a delicious watermelon, look for bee stings: small holes in the melon with dried sap and a bit of scarring around it. Bee stings indicate a sweet melon!
Once you got yourself a delicious watermelon, use a very sharp and large knife to cut the melon into 6 or 8 large wedges, depending on the size of your melon.
Cut each wedge into 2-cm (or about 1-inch) thick slices.
If you want to put the watermelon pops on a stick, use a smaller knife to make a small cut in the rind of each of the watermelon slices and carefully insert a popsicle stick in each cut.
Arrange the watermelon pops on a baking sheet (in a single layer) and freeze until frozen.
Once frozen, sprinkle with chopped mint leaves and serve!
Notes
Once frozen, the pops can be placed in a large freezer bag and stored in the freezer for up to two months.
Recipe by The Tough Cookie at https://thetoughcookie.com/2014/08/21/super-healthy-watermelon-pops/