How do you stop rough idling and sputtering on a 92 Pontiac firebird?

Could be a number of things. Start by performing any maintenance items that are due and listed in you owner's manual. If you have not done so, replace all vacuum hoses, including the brake booster hose. Check the booster for a cracked or leaking diaphragm. Replace the hoses one at a time to avoid getting confused. If it idles rough but runs good under acceleration, then a vacuum leak is probable.