When a voiceless sound appears right before the “s,” you pronounce the “s” with a /s/ sound, as in snake. Some examples are: likes, apps, cats.
When a voiced sound appears right before the letter “s,” you pronounce the “s” with a /z/ sound, as in zoo. Some examples are: kids, dogs, tables, exams, cans, scissors.
Remember that all vowel sounds are voiced, so words with -es spellings will also be pronounced with a /z/ sound. Some examples are: heroes, boxes, pushes.
My student learned this concept very quickly yesterday. I hope that was just as easy for you too!