There are non-compound and compound vowels, and the actual pronunciation of the compound ones are usually not simply saying the non-compound ones in sequence. However, this post tries to make things easier by telling you how to pronounce the non-compound ones and simply pronounce them in sequence for compound ones.
The non-compound vowels are:
a o e i u ü
ü is the hardest one, so we go with that first, because you cannot be too wrong with other vowels but can be completely wrong with that, for the reason that no phonemes in English sound even remotely like it. If you also speak German or French, it is the same sound as the “ü” in “über” in German or “u” in “tu”, “juge” or “su” in French. To describe it directly, it is the sound when your tongue is at the same position as when saying “ee” in “bee” but your lips are round like when saying “oo” in “foo”.
Among the other non-compound vowels, a can be simulated by one single sound no matter where it appears: pronounce a like in “father”, not like in “bad”. The rest each have their caveats.
i can be pronounced as “ee” like in “need” most of the time. The exceptions are when it follows z, c, s, zh, ch, sh and r: in those cases, quoting from Wikipedia, it is “a buzzed continuation of the consonant.” That is, after you have pronounced the consonants, do not move your lips or tongue while keeping your vocal cords vibrating.
u itself can also be almost always simulated by one single pronunciation: as “oo” in “food”.
For e, always pronouncing it as the schwa, i.e. like “a” in “ago” or the “uhh” sound you make when hesitating, is close enough, although not as close as the other vowels described as “close enough” above. If you want to sound a bit more correct, in ie and üe it should be pronounced as a sound that is in the middle of “e” in “bet” and “i” in “bid”. For example, ie sounds like reversed “ey” in “hey”.
o usually appears with other vowels, and can also be simulated by a single sound. For North American English speakers, pronounce it like “ough” in “thought”. For British English speakers, pronounce it like “augh” in “caught” or “o” in “office”.
For the compound vowels like ao, ou, ian and so on, pronouncing their consisting non-compound vowels will make it likely recognizable.
Three exception are iu, ui and un. These are actually abbreviations: iu expands to iou, ui expands to uei, un expands to uen.
The first is when it follows y. As stated above, the consonant y is not always pronounced like “y” in English, and the cases where it is not are yu combinations, which represents the semivowel corresponding to ü just like how the usual y corresponds to i and w corresponds to u.
The second is about the transliteration of ü. Since ü is not in the English alphabet, the vowel ü needs to be represented differently when transliterating to English. Thanks to the rule mentioned above that ü is written as u in a lot of places, the only places where ü really appears are following l and n, such as lü or nüe. And among these places, the only two that require particular treatment during the transliteration is lü and nü. In names, they are usually transliterated as lv/nv casually (like in daily typing) or lyu/nyu officially (like on passports), and sometimes (unfortunately), the ambiguous lu/nu.