Press your lips together. Air flows through your nose. Vocal cords vibrate.

Americans pronounce magnetism as MAG-nuh-tuh-zuhm (/ˈmægnəɾəzəm/). In "magnetism", the "t" between vowels sounds like a quick "d" — the tongue briefly taps the ridge behind the upper teeth. This is called the Flap T, a hallmark of natural-sounding American speech. It comes out as MAG·nuh·tuh·zuhm. Stress falls on the first syllable — keep everything else short and quick. You'll hear it in sentences like "Magnetism is a force that can attract or repel objects".
Record yourself saying "magnetism" and play it back. The mic stays on your device — nothing's uploaded.
4 syllables, 10 sounds. Tap a syllable to jump to its row, then explore each sound's mouth shape and how it's made.
Press your lips together. Air flows through your nose. Vocal cords vibrate.

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.

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

The textbook way isn't wrong — it's just not how anyone actually says it.
In "magnetism", the "t" between vowels sounds like a quick "d" — the tongue briefly taps the ridge behind the upper teeth. /t/ or /d/ becomes a quick tap [ɾ] — sounds like a soft D. The tongue briefly taps the ridge behind the upper teeth.
In "magnetism", the "t" is not released — the articulators get into position but hold without the burst of air. Air stops but there's no release burst — the articulators hold position.
In "magnetism", the short unstressed vowel before "m" disappears — the schwa is absorbed and the "m" becomes the syllable nucleus on its own. Schwa is absorbed — consonant becomes the syllable nucleus.
Stress falls on the first syllable, not the others. Stretch MAG — keep everything else short and quick.