Here's how to do it:
1. Start with your lips rounded and slightly protruded, like you're about to whistle.
2. Position your tongue slightly lower in your mouth than for a short "u" sound (like in "cup").
3. Let the air flow out steadily from your mouth, maintaining the rounded lip shape.
4. Hold this sound for as long as you need to.
Think of words like:
* "flute"
* "tune"
* "food"
* "rule"
Practice:
* Say the sound in isolation, focusing on maintaining the lip shape and airflow.
* Say words that contain the long U sound and exaggerate the vowel sound.
* Read aloud a passage containing words with the long U sound, paying attention to holding the sound for a longer duration.
Remember that the length of the vowel sound will depend on the context of the word and the rhythm of the sentence.