Lips
Slightly rounded and relaxed, mouth mid-open. Hold this shape steady: unlike the gliding vowels, this one barely moves.
The American "er": one steady, r-coloured vowel. The voice and the r shape happen at the same time, from start to finish.
Example words: nurse, bird, her, work, first
/ɝ/ is the stressed form (nurse, bird, her); /ɚ/ is the light, unstressed one at the end of words like letter and doctor. The tongue shape is the same.

Slightly rounded and relaxed, mouth mid-open. Hold this shape steady: unlike the gliding vowels, this one barely moves.
Curl the tip up and back, or bunch the middle of the tongue upward. Either works. The tip must never touch the roof of the mouth: it floats.
Can be held as long as you like. Keep the r colour running for the entire vowel, not just the end.
/ɚ/ (letter, doctor) has its own lesson:
Hold "errrr" for three full seconds. The r colour should stay even the whole time, and your tongue tip should never touch the roof of your mouth. If the sound flickers, the tip is tapping.
Dropping the r entirely so "bird" sounds like "bud", or tapping and trilling the tongue against the roof. There is no contact and no vibration: just a held shape.
🔊 plays the app's native voice. 🎤 records you so you can play it back next to it. Your recording stays in your browser.