Wet the rim of your serving glass with a cut lime or grapefruit, and dip the rim into the salt, coating it all the way around.
If using Fresh Citrus Juices
Fill the serving glass with ice and add in the tequila.
Add in the lime juice, grapefruit juice, and jalapeño syrup.
Add the jalapeño slices to the glass and stir with a bar spoon.
Top with seltzer.
Add the grapefruit slice to the side of the glass and serve.
If using Soda or a Pre-made Mixer
If you are using a plain grapefruit soda, add your jalapeño slices to the bottom of the serving glass and muddle them with a cocktail muddler. You can skip this step if your mixer already has jalapeño in it.
Add ice to the serving glass and add in the tequila.
Top with the soda or mixer. If your mixer doesn't have lime, add in the lime juice too. Give it a quick stir with a bar spoon.
Add the jalapeño slices and fresh grapefruit slice to garnish, and serve.