To make homemade Tajin seasoning, in a medium serving bowl add flaky sea salt and lime zest. Use your fingers to rub the salt and lime zest together. Add the chili powder and combine with a fork. Use this bowl to continue on with the recipe.
In the same mixing bowl add avocado, watermelon, red onion, cilantro, Tajin, lime juice, and jalapeño, if using. Fold together gently until combined. This is best served immediately so the avocado doesn’t get brown.