What is Youon?
Youon (拗音) literally means 'twisted sound'. It refers to diphthongs in Japanese that contain a 'Y' sound in the middle. Despite its fancy name, Youon naturally exists in English. Examples are the letter 'Q', 'thank you', 'watch you', British pronunciation of 'new', etc. But of course, a lot of Japanese Youon combinations do not exist in English.
I recommend using 'thank you' to help you understand how to pronounce Youon. When we pronounce the 'k you' part, our mouths are ready to pronounce the 'k' sound but do not really pronounce it. Instead, we pronounce 'you' immediately and this 'you' is thus combined with the not yet pronounced 'k' sound to form 'kyou'.
In Japanese, the mechanism is more or less the same. Youon are formed by two Hiragana characters or two Katakana characters, such as きゃ, みょ and りゅ. The Kana on the left hand side can only be きぎしじちにひびぴみり or their Kanakana counterparts. In other words, only Kana that end with an 'i' sound can form the left part of Youon. On the right hand side, we can only use Kana that begin with 'y', namely やゆよ. And やゆよ in Youon must be shorter and smaller than the Kana on the left. (Compare やゆよ and ゃゅょ) When you pronounce Youon, your mouth should be about to pronounce the Kana on the left, e.g. に, without actually pronouncing it out. Then you immediately pronounce 'ya', 'yu' or 'yo' to form 'nya', 'nyu' and 'nyo'. The point is that the 'n' must be combined with 'y' to form 'ny'. Youon are diphthongs that have only one syllable. If you pronounce Youon with two syllables, then you are wrong.
Youon is very common in Japanese. But it rarely exists outside loanwords that have Chinese origin or modern European loanwords. In other words, traditionally Japanese did not have Youon. It is believed that when ancient Japanese people imported Chinese vocabulary into their language, the pronunciations of those Chinese words were so different from Japanese. So people needed to invent a new pronunciation system to pronounce those loanwords. Ironically, nowadays quite a lot of Chinese speakers have a hard time learning Youon.
How to write:
The stroke order illustrations are made by D.328, used under CCSA3.0.