This is a pronunciation you'll have to learn to hear, as well as pronounce. The thick "th" (ð) is the same as the thin th (θ), but voiced. The tongue is still between the teeth. The sounds "z" and "d" sit behind the teeth... and native speakers can definitely hear the difference!

/ ð / * is a voiced dental fricative. * if you feel your throat, you should be able to detect the vibrations of a voiced consonant. * you can also feel your tongue tip acting against your upper front teeth, thus making /ð/ a dental sound. * The fact that air is forced between the tongue and the teeth means that it is a fricative consonant. * In normal English spelling, no distinction is made between this sound and the softer /th/ sound used in " think " * note: the /ð/ sound can be spelt : th as in the, that , father. the as in bathe , breathe.