Place the very tip of your tongue slightly between your teeth. Blow air gently around it without voicing.

Americans pronounce theatre as THEE-uh-ter (/ˈθiəɾər/). In "theatre", the "t" between vowels sounds like a quick "d" — the tongue briefly taps the ridge behind the upper teeth. This is called the Flap T, and it's one of the defining features of casual American English. It comes out as THEE·uh·ter. Stress falls on the first syllable — keep everything else short and quick. You'll hear it in sentences like "The theatre theme was thought provoking".
Record yourself saying "theatre" and play it back. The mic stays on your device — nothing's uploaded.
3 syllables, 5 sounds. Tap a syllable to jump to its row, then explore each sound's mouth shape and how it's made.
The textbook way isn't wrong — it's just not how anyone actually says it.
In "theatre", the "t" between vowels sounds like a quick "d" — the tongue briefly taps the ridge behind the upper teeth. /t/ or /d/ becomes a quick tap [ɾ] — sounds like a soft D. The tongue briefly taps the ridge behind the upper teeth.
Stress falls on the first syllable, not the others. Stretch THEE — keep everything else short and quick.
Don't pronounce the first syllable too fully. The unstressed syllable reduces to a schwa — the lazy "uh" sound — in casual speech.
Americans use a relaxed retroflex R — the tongue curls back rather than rolling. The R is one continuous sound with the vowel before it, not two separate sounds.