Press your lips together, add vocal cord vibration, then release.

Americans pronounce buck as BUHK (/bʌk/). It's 3 sounds in 1 syllable. Ends in a crisp stop consonant — in casual speech the puff after it often disappears.
Record yourself saying "buck" 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.