To me, the sound would be very similar to the one in "Few".
If you look at the IPA of "iu" in Chinese, which you can find in wiki ([iɤu], with some decorations I'm not able to reproduce here) or SAMPA-C ([iəu], "iu" is spelled as "iou" in the document), you'll see that unlike the [jʊ] in "Few", we used [i], but not [j] and there's another (transitional) vowel in the middle.
'iu' is made up of the 'i' and 'ou' sound in Chinese. (That's why it is spell as 'iou' in the SAMPA-C document, and also in the pinyin "you".) So you can try to produce "iu" from 'i' and 'ou'. Sorry I'm not able to describe how to produce the sound literally. My suggestion, like others have made in the comment, is to listen to how natives say it. And if you can find a native, let him/her correct your pronunciation.
NOTE: The two versions of IPA came from the two sources may come from the variation of how different people produce it, and may come from the notation variations of IPA, or both.