Two sounds close enough to trade places: the “zh” of vision and the “z” of zoo. Swap one for the other and a different word comes out — this page is how to keep them apart, in both directions.
Pull your tongue back from the teeth, push your lips slightly forward, and keep the voicing.
“Your buzz is front-row when it should be mid-theater! Slide the tongue back, round the lips, zhh like a French film!”Nunzio, when he hears it
You cannot hear your own swap. That is the whole problem.
Nunzio records you reading a sentence, scores every sound in it on your phone — nothing is uploaded — and tells you which of these two actually came out. Nunzio — Accent Coach is coming to the App Store and Google Play.