Both /əˈdʌlt/ (a-DULT) and /ˈæd.ʌlt/ (AD-ult) are verified for US and UK English, with no stated preference in either variety. Choose one stress pattern that matches your target variety and use it consistently. For /əˈdʌlt/, say a light uh, then strong DULT.
Mouth movements
Start with a relaxed uh: let your mouth rest almost closed, make a very short vowel, and give it no stress at all.
Move straight into DULT. Press your tongue tip up behind the top teeth for the d, then release it with a burst of air.
Land the strong vowel like cup: drop your jaw a little, keep the tongue low and relaxed, and finish with the tongue tip touching the ridge for the final l. Say it twice as loudly and slightly longer than the first part.
Contrast check beside the strong vowel: a-DULT uses the low vowel of cup; AD-ult starts with the spread vowel of cat. Open your jaw more for the cup vowel, and keep the lips unrounded in both.
Whole word in one breath: an a-DULT student, with a light first syllable and a firm second one.
Say it in a phrase
Contrast check beside the strong vowel: a-DULT uses the low vowel of cup; AD-ult starts with the spread vowel of cat. Open your jaw slightly more for the cup vowel, and keep the lips unrounded in both.
Try the second-syllable stress in a sentence: The a-DULT class starts at seven. Keep the first syllable light and the second one firm and long.
Try the first-syllable stress in a sentence: The AD-ult class starts at seven. Press down firmly on AD, then let ult fade quietly away.
Both stress patterns are verified for American and British English, so pick the one that matches your target variety and reuse it consistently. Alternating between them can confuse listeners more than either choice alone.