Raise the back of your tongue to touch the soft palate. Add vocal cord vibration, then release.

Americans pronounce garnished as GAR-nuhsht (/ˈgɑrnəʃt/). Stress falls on the first syllable — keep everything else short and quick. You'll hear it in sentences like "She garnished the plate with fresh parsley and a lemon wedge".
Record yourself saying "garnished" 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.
Touch the tip or front edge of your tongue to the roof of your mouth behind your teeth. Air flows through your nose.

Relax your lips, jaw, and tongue completely. Drop your jaw slightly and keep the tongue neutral.
Flare your lips and lift the mid-front tongue close to the roof of your mouth. Blow air through 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.

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 GAR — 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.