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

Americans pronounce postpone as poh-SPOHN (/poʊˈspoʊn/). Stress falls on the second syllable — keep everything else short and quick. You'll hear it in sentences like "I am afraid I need to postpone our dinner due to a prior commitment".
Record yourself saying "postpone" and play it back. The mic stays on your device — nothing's uploaded.
2 syllables, 6 sounds. Tap a syllable to jump to its row, then explore each sound's mouth shape and how it's made.
Place your tongue tip near the roof of your mouth behind your top teeth. Push air through the narrow gap. No voicing.

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

Start with your mouth slightly open, then close your jaw slightly as your lips round. Shift your tongue back slightly, then stretch the back up.
Touch the tip or front edge of your tongue to the roof of your mouth behind your teeth. Air flows through your nose.

The textbook way isn't wrong — it's just not how anyone actually says it.
Stress falls on the second syllable, not the others. Stretch SPOHN — keep everything else short and quick.