Place chiles in a food processor and pulse to finely chop (stop before they are mush). Or, wearing rubber gloves to protect your hands, use a cleaver or sharp knife to mince chiles on a cutting board.
Transfer minced chiles (with their seeds) to a glass or plastic container and add fish sauce. Cover and store in refrigerator. The sauce will keep indefinitely, losing chile heat over time; top it up with extra chiles or fish sauce when it runs low. Serve in small individual condiment bowls.