This maybe deserves a better name, but this is less a recipe that a general thing I do a few times every summer.

Preparation: Next time you have a watermelon cut up a bunch of it and throw it into a zip lock bag and throw into the freezer.

For 2:
Throw enough watermelon cubes into a blender to make enough for two tall glasses. Less than half full? Sorry, I just eyeball this, but really the proportions aren't critical, and I always adjust to taste anyway.

Add
3 ounces of gin
1 ounce of cocchi americano
Juice of 1 and a half limes (maybe start with 1 and adjust to taste?)
A handful of fresh mint leaves
A few splashes of orange bitters (I like Regan's)
Blend it up and enjoy!

I tend to vary this, but this is about my default. I might use rum. I might use Campari instead of cocchi americano or half/half. I might lighten it up with some tonic water or some other fizzy thing that's on hand.

Fresh watermellon and some ice cubes works in a pinch, but really the frozen watermelon is great to have in the freezer, can be used in other drinks, and lasts well (what I used tonight is from last summer).