Raise the back of your tongue to touch the soft palate (velum). Stop the air, then release.

Americans pronounce commerce as KAH-mers (/ˈkɑmərs/). Stress falls on the first syllable — keep everything else short and quick. You'll hear it in sentences like "E-commerce sales have surged as shopping habits continue to evolve".
Record yourself saying "commerce" and play it back. The mic stays on your device — nothing's uploaded.
2 syllables, 5 sounds. Tap a syllable to jump to its row, then explore each sound's mouth shape and how it's made.
Raise the back of your tongue to touch the soft palate (velum). Stop the air, then release.

Relax your lips and drop your jaw significantly. The tongue tip lightly touches behind the bottom front teeth and the back part of the tongue presses down a little to create more dark space in the back of the mouth.

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 KAH — 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.