Drop your jaw slightly with relaxed lips. Touch the tongue tip behind the bottom front teeth and arch the top-front toward the roof.

Americans pronounce imposed as ihm-POHZD (/ɪmˈpoʊzd/). Stress falls on the second syllable — keep everything else short and quick. You'll hear it in sentences like "International sanctions were imposed in response to the violations".
Record yourself saying "imposed" 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.
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.
Same position as S, but add vocal cord vibration. Feel the buzz.

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

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 POHZD — keep everything else short and quick.