How are you is /ˌhaʊ ər ˈju/ in American English and /ˌhaʊ ə ˈju/ in British English, per Cambridge. Stress falls on how and you; are takes its weak form.
Respelling like how-uh-you is only approximate. Treat the phonemic transcription above as the precise reference.
Start how with a breathy h: open your mouth slightly and let air pass before the vowel, as in hot.
Let are stay light in the middle. In American speech the r sound remains at the end; in British speech it usually fades, leaving a longer vowel. In fast connected speech you may hear the reduced form how're you, where are shrinks almost to a single vowel.
Finish on you with a clear, round-lipped vowel, then practise the whole phrase in a simple line like Hi, how are you?
A caution: respellings such as how-uh-you are only approximate and should not be treated as precise phonetics. A phonemic alphabet exists because ordinary spelling cannot represent every sound; until a verified transcription with stress marks is available, use these mouth-movement cues as your guide.
Say it in a greeting: Hi, how are you? Keep the stress on how and you, with are light in the middle.
A useful contrast: in American speech the r in are is audible even before a pause, while in British RP it usually disappears unless a vowel sound follows. Compare how are you said each way, keeping the same stress pattern.
Practise the same r difference with far away versus far from. In far away the r stays in both accents because a vowel follows. In far from it fades in British speech, so the vowel lengthens slightly instead.
For the middle of the phrase, let are shrink to a quick vowel between how and you. Your tongue barely stops moving: the vowel carries you through, then the lips round for you.
Respellings like how-uh-you are only approximate. Treat the phonemic transcription as the precise reference, and use the mouth-movement cues above as your practice guide.