Push a stream of air from your throat through your open mouth. No tongue or lip contact.

Americans pronounce happiness as HA-pee-nuhs (/ˈhæpinəs/). Stress falls on the first syllable — keep everything else short and quick. You'll hear it in sentences like "I experienced a moment of pure happiness during the ceremony".
Record yourself saying "happiness" and play it back. The mic stays on your device — nothing's uploaded.
3 syllables, 7 sounds. Tap a syllable to jump to its row, then 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 behind your teeth. Air flows through your nose.

Relax your lips, jaw, and tongue completely. Drop your jaw slightly and keep the tongue neutral.
Place your tongue tip near the roof of your mouth behind your top teeth. Push air through the narrow gap. No voicing.

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 HA — keep everything else short and quick.
Don't pronounce the second syllable too fully. The unstressed syllable reduces to a schwa — the lazy "uh" sound — in casual speech.