Got the idea from this recipe that I made last year for Pickled jalapenos. So this time I did pickled Serrano peppers instead.
Original recipe: recipe
Pack Serrano slices into jar. Bring vinegar to a boil and pour enough over slices to cover.
Allow to cool on the counter for a couple of hours. As the Serrano slices settle the vinegar may need to be topped up in order to keep them covered.