First make your watermelon juice - see below. This can be done ahead and refrigerated.
Put some salt and a little chili on a small plate and mix. Rub the rim of a glass with a slice of lime then dip in the salt-chili mixture to form a salt rim.
Put the jalepeno slices in a cocktail shaker and muddle a little to release a little juice. (No need to do this if you plan to shake a lot or want less spicy)
Add the watermelon juice, tequila, tripe sec and lime juice along with a few pieces of ice. Shake a minute until well chilled.
Pour into the prepared glass, add ice if you like to chill further, and serve.
Notes
To make watermelon juice, puree around 1 cup (225g) of watermelon chunks then strain through a relatively fine strainer to remove seeds and large pieces of pulp and you should get a little over the amount of juice needed for this.