How do you write the short form of "you all"?

The short form is pronounced as "yoll", but what is the actual spelling? Is it "y'all"? Any official mention of the spelling? Example useage: "Where are you-all going". Pronounced as "Where are yoll going". Would it be written as "Where are y'all going"?

Would it be written as "Where are y'all going"?

Yes. The apostrophe is used to stand for the letters left out.

In formal settings, "you all" is never contracted to "y'all" (or hyphenated). It is pronounced just like it looks: "you all", with no contraction.

Furthermore, "you" has come to function as both a singular and plural second-person pronoun. "You all" is typically used only when the number is ambiguous, or to emphasize that you're talking about a group of people.

Compare "You are my friend." with "You are my friends." The plurality of the object tells you whether the pronoun is singular or plural, without ever resorting to "you all".

"Y'all" is a construction limited to a couple of American dialects, particularly Southern and Western speakers. I've heard it pronounced to rhyme with "wall" (approximately--the vowel gets slightly drawn out/duplicated). There is a joke poking fun at non-prestige dialects to the effect of "Remember, y'all is singular, all y'all is plural." There is actually a bit of truth to that for the heaviest examples of the dialect. In typical usage, I believe, "y'all" is used as both a singular and plural pronoun--nicely mirroring "you" in the standard dialect.