Drop the jaw noticeably. Keep the body of the tongue low and forward, and don't let the back of the tongue raise toward the soft palate. Pull the lip corners back slightly, almost a starting smile.

Americans pronounce afternoons as a-fter-NOONZ (/ˌæftərˈnunz/). Stress falls on the third syllable — keep everything else short and quick. You'll hear it in sentences like "The discussion section meets once a week on Friday afternoons".
Record yourself saying "afternoons" and play it back. The mic stays on your device — nothing's uploaded.
3 syllables, 8 sounds. Tap a syllable to jump to its row, then explore each sound's mouth shape and how it's made.
Lift your bottom lip to touch the very bottom of your top front teeth. Blow air through this contact point without voicing.

Touch the tip or front edge of your tongue to the roof of your mouth just behind your teeth. Keep your jaw relaxed. Stop the air, then release with a puff.

Relax your mouth and lift the tongue back and up. Keep the lips neutral.

Touch the tip or front edge of your tongue to the roof of your mouth behind your teeth. Air flows through your nose.

Round your lips into a tight circle. Let your tongue rest in the middle of your mouth, slightly raised.
Touch the tip or front edge of your tongue to the roof of your mouth behind your teeth. Air flows through your nose.

Same position as S, but add vocal cord vibration. Feel the buzz.

The textbook way isn't wrong — it's just not how anyone actually says it.
Stress falls on the third syllable, not the others. Stretch NOONZ — keep everything else short and quick.
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.