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

Americans pronounce preheat as pree-HEET (/ˌpriˈhit/). Stress falls on the second syllable — keep everything else short and quick. You'll hear it in sentences like "I need to preheat the oven to three hundred fifty degrees Fahrenheit".
Record yourself saying "preheat" 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.

Curl or bunch your tongue without letting the tip touch the roof of your mouth. Brace the sides of your tongue against your upper back teeth, and round your lips slightly.
Pull the corners of your lips back slightly. Arch the middle-front of your tongue high toward the roof of the mouth.

Push a stream of air from your throat through your open mouth. No tongue or lip contact.

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

Touch the tip or front edge of your tongue to the roof of your mouth just behind your teeth. Keep your jaw relaxed. Stop the air, then release with a puff.

The textbook way isn't wrong — it's just not how anyone actually says it.
In "preheat", the "t" 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 second syllable, not the others. Stretch HEET — keep everything else short and quick.