First, infuse the tequila. Cut the cap off the jalapeño and discard it. Cut the jalapeño in half length-wise and scrape out the seeds and white pith with a small metal spoon. Discard the seeds and pith. Place the jalapeños in a sterile glass container and add the tequila. Seal the container and allow to rest for four hours or overnight in a cool dark place. If you're doing a short 4-hour infusion, vigorously shake the container every 30 minutes for about 10 seconds. If you're infusing it overnight, you don't need to shake it. You can allow it to infuse for up to 1 week, keeping in mind that the heat will not increase much between the overnight infusion and the weeklong infusion, but the flavor of the jalapeño pepper itself will. Once the desired flavor intensity is reached, remove the jalapeño from the container and discard it.