Place the salt, sugar, and garlic in a sterilized quart jar. Pack the peppers firmly into the jar, with the skins facing out. Pour in the vinegar. Fill the jar to within ½-inch of the top with boiling water. Seal and process in a boiling-water bath for 5 minutes; start counting the processing time when the water starts to boil. Set aside to cool undisturbed for 12 hours.