Add sugar and water to a small saucepan. For a less spicy syrup, remove ribs and seeds from jalapeno then add to the saucepan. For a more spicy syrup, slice jalapeno then add - seeds and all - to the saucepan. Bring mixture to a simmer over medium-high heat, stirring occasionally, then cook until sugar is dissolved, 1-2 minutes. Remove pan from heat then let cool - the longer the jalapeno stays in the syrup, the spicier it will be. Strain syrup through a fine mesh sieve into a mason jar or container then chill. Can be done up to 2 weeks ahead of time.
Watermelon Jalapeño Margaritas:
Add watermelon juice, tequila, lime juice, and jalapeno simple syrup to a rocks or cocktail glass then stir to combine. Fill glass to the top with ice then serve with fresh jalapeno slices. Alternatively you could add the ingredients into a shaker, fill it halfway up with ice, then shake to chill.
Notes
Watermelon juice is simply blended or juiced watermelon that's been strained through a fine mesh sieve. Plan on roughly 1/2 cup volume loss when blending or juicing (ie, 2 cups chopped watermelon will give you approximately 1-1/2 cups watermelon juice.) I recommend making the watermelon juice no more then a few hours ahead of time so it's very fresh. Alternatively you can