Rinse the jalapeños, slice them into rounds, and peel the garlic cloves. The peppers should look fresh and glossy, and even slices will pickle more evenly.

Pour the vinegar and water into a medium sauce pot, then add the salt and sugar and bring it to a boil over medium-high heat while stirring. Keep stirring until the brine goes from cloudy to clear, which shows the salt and sugar have dissolved.

Add the sliced jalapeños and garlic as soon as the brine boils, then turn off the heat, cover the pot, and let everything sit for 30 minutes, stirring occasionally. The hot brine does the pickling here, and a sharp vinegary smell is normal, so a little ventilation helps.

Transfer the peppers and brine to a non-reactive airtight container after the 30-minute soak. The jalapeños will shift from bright green to a more muted olive green, which is the sign they are pickling properly.