"用咝音发" can be translated as "pronounce with a sibilant sound" or "articulate using a sibilant" in English. Here, "sibilant" refers to a hissing or whistling sound made when pronouncing certain consonants, like /s/, /z/, /ʃ/ (as in "she"), or /ʒ/ (less common in English, but found in words like "vision" in some accents).
For a more natural-sounding instruction in a language-learning context, you might also say something like: "Pronounce it with a hissing sound" or "Make a sibilant sound when you say it."