Youon kana are the combination of two symbols, that make one sound. The first symbol is represented by a regular sized kana that ends in the sound i. For example, び き ち じ all end in the sound i. The second symbol is a smaller version of one of the three sounds ゃ ゅ ょ.
When these characters are put together the sound blends together and is pronounced as one sound.
きゅ is pronounced kyu
びょ is pronounced byo
ちゃ is pronounced cha
じゃ is pronounced ja
As you can see some sounds don't follow the consonant + (ya, yu, yo) pattern. Sounds that start with J = ja ju jo, S = sha shu sho, C = cha chu cho.
You cannot make youon kana from every sound. Sounds that begin with the sounds d, z, w, n or vowels don't make youon kana.
Practice by translating the romaji to kana on a blank paper.
NYA RYO CHU GYO
KYO BYU JU SHO
NYO PYA SHA
JO RYA MYU KYO