Touch the tip of your tongue to the roof of your mouth just behind your teeth. Add vocal cord vibration as you release.

Americans pronounce dumped as DUHMPT (/dʌmpt/). You'll hear it in sentences like "Someone must have dumped the luggage in the trunk".
Record yourself saying "dumped" and play it back. The mic stays on your device — nothing's uploaded.
1 syllable, 5 sounds. Explore each sound's mouth shape and how it's made.
Touch the tip of your tongue to the roof of your mouth just behind your teeth. Add vocal cord vibration as you release.

Relax your lips, jaw, and tongue completely. Drop your jaw slightly and keep the tongue neutral.
Press your lips together. Air flows through your nose. Vocal cords vibrate.

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

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.
In "dumped", 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.