Raise the back of your tongue to touch the soft palate (velum). Stop the air, then release.

Americans pronounce cornfield as KORN-feeld (/ˈkɔrnˌfild/). The L in "cornfield" 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, and it's one of the defining features of casual American English. It comes out as KORN·FEELD. Stress falls on the first syllable — keep everything else short and quick. You'll hear it in sentences like "The scarecrow keeps the birds away from the cornfield".
Record yourself saying "cornfield" and play it back. The mic stays on your device — nothing's uploaded.
2 syllables, 7 sounds. Tap a syllable to jump to its row, then explore each sound's mouth shape and how it's made.
Raise the back of your tongue to touch the soft palate (velum). Stop the air, then release.

Start with the 'aw' jaw drop and rounded lips. Pull the tongue back and up while keeping the lips rounded for the R.
Touch the tip or front edge of your tongue to the roof of your mouth behind your teeth. Air flows through your nose.

Lift your bottom lip to touch the very bottom of your top front teeth. Blow air through this contact point without voicing.

Pull the corners of your lips back slightly. Arch the middle-front of your tongue high toward the roof 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.

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.
The L in "cornfield" 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.
In "cornfield", the "d" is not released — the articulators get into position but hold without the burst of air. Air stops but there's no release burst — the articulators hold position.
Stress falls on the first syllable, not the others. Stretch KORN — 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.