When s is placed after ce, ge, se or ze: an extra syllable /iz/ is added to the spoken word
do the same with es when it’s placed after ch, sh, ss or x
Otherwise it is pronounced /z/
There is a small trick:
if you don’t know or feel hard to digest those on, just put a finger on your fauces when pronounce. If it tremble then you pronounce s like /z/, if not, look at the 2nd situation with p, k and f.