Round your lips into a tight circle. Lift the back of your tongue toward the soft palate and add voice.

Americans pronounce wallpaper as WAHL-pay-per (/ˈwɑlˌpeɪpər/). The L in "wallpaper" is a dark L — the back of the tongue rises toward the soft palate, adding a small "uh" quality before the L. This is called the Dark L vs Light L, a small move that separates 'classroom' from 'native'. It comes out as WAHL·PAY·per. Stress falls on the first syllable — keep everything else short and quick. You'll hear it in sentences like "She demolished the old wallpaper and prepared the walls for painting".
Record yourself saying "wallpaper" 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.
Round your lips into a tight circle. Lift the back of your tongue toward the soft palate and add voice.

Relax your lips and drop your jaw significantly. The tongue tip lightly touches behind the bottom front teeth and the back part of the tongue presses down a little to create more dark space in the back of the mouth.

Keep the tongue tip down and pull the back of the tongue up toward the throat. The 'dark' sound comes from the back.

The textbook way isn't wrong — it's just not how anyone actually says it.
The L in "wallpaper" is a dark L — the back of the tongue rises toward the soft palate, adding a small "uh" quality before the L. Dark L adds a small schwa-like "uh" before the L. The back of the tongue lifts toward the soft palate.
Stress falls on the first syllable, not the others. Stretch WAHL — keep everything else short and quick.
Americans use a relaxed retroflex R — the tongue curls back rather than rolling. The R is one continuous sound with the vowel before it, not two separate sounds.