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.

Americans pronounce tube as TOOB (/tub/). You'll hear it in sentences like "Cute music usually uses a huge tube".
Record yourself saying "tube" and play it back. The mic stays on your device — nothing's uploaded.
1 syllable, 3 sounds. 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 just behind your teeth. Keep your jaw relaxed. Stop the air, then release with a puff.

Round your lips into a tight circle. Let your tongue rest in the middle of your mouth, slightly raised.
Press your lips together, add vocal cord vibration, then release.

The textbook way isn't wrong — it's just not how anyone actually says it.
In "tube", the "b" 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.