Press your lips together to stop the air, then release. No vocal cord vibration.

Americans pronounce posters as POH-sterz (/ˈpoʊstərz/). Stress falls on the first syllable — keep everything else short and quick. You'll hear it in sentences like "He collects vintage movie posters from the golden age of cinema".
Record yourself saying "posters" and play it back. The mic stays on your device — nothing's uploaded.
2 syllables, 6 sounds. Tap a syllable to jump to its row, then explore each sound's mouth shape and how it's made.
Place your tongue tip near the roof of your mouth behind your top teeth. Push air through the narrow gap. No 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.

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 first syllable, not the others. Stretch POH — 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.