Place the very tip of your tongue slightly between your teeth. Blow air gently around it without voicing.

Americans pronounce thumb as THUHM (/θʌm/). It's 3 sounds in 1 syllable. Short, clean, and front-of-mouth — every sound earns its place.
Record yourself saying "thumb" 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.