Minimal Pair /ð/ and /z/
Below is a list of words that vary only by one having the sound /ð/ and the other the sound /z/.
You can use this list to practise the sounds, or as a list of words to be careful in pronouncing.
/z/ is pronounced with the mouth in the same position as with /s/, but using the vocal cords. The tongue is well inside the mouth, and it can also help to spread the lips thin and wide to make the sound unlike /ð/.